The leader of the kids is named Ian. After becoming friends with Luka and Sylvia, he and his group often hang out with them, despite not knowing magic.
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{THIRD PERSON POV}
"Hey Luka," Ian asked suddenly one summer day. "Your mom is a celestial spirit mage, isn't she? I think I've seen one of those keys. A golden one."

"That's impossible." Luka replied. "There are only twelve golden keys. Mom has nine and Aunt Yukino has the other two. There's only—" his eyes widened, "—one... unaccounted... for..."

"With little blue squiggles?"

Luka slowly turned to face Ian. "Where?"

Ian scratched his head sheepishly. "In the ocean, while I was on vacation. I was scuba diving near my family's beach house when I spotted something small glowing gold on the sea floor. I felt like it was calling out for someone. Stupid, eh?"

"Do you have the key?" Luka asked intently.

Ian shook his head. "Nah, I couldn't get close. The current was too strong, sweeping me away. I felt like that key was waiting for a specific person to pick it up. Besides, it was too deep for my oxygen lacrima to withstand the pressure anyway. Right on the edge of the shelf, next to the ocean proper."

Luka was trembling. Could it really be? It had been years since his mom broke her most powerful key, and she still cried about it sometimes.

"Ian, if I pack some stuff, will you take me to this beach house of yours?"

"Sure." Ian nodded. "Give me twenty minutes to get a carriage ready. I'll pick you up at the guild hall."

"Okay." Luka nodded, and the two boys parted ways.

Luka ran to his house first, pulled out the red suitcase he used for missions, and packed about a week's worth of clothes.

Then he ran to the Fairy Tail guild hall. When he entered, Luka saw everyone gathered around his mom. She was sobbing.

"It's gone! I know I had the remnant after our last mission, but now it's just disappeared!"

"We'll find Aquarius's key, Lu-chan." Levy said kindly, rubbing her shoulders. "We'll search every nook and cranny until we find it."

Luka knew he couldn't tell his mom what he was attempting, and this sealed the deal. He couldn't get Lucy's hopes up in case Ian was wrong.

"Luka?" Natsu recognized his son's scent and looked up. "What's with the suitcase?"

"I'm running away." Luka replied.

"Okay, have fun—hold up."

Luka ignored his father and turned back to leave. Ian was waving at him in front of a carriage. At the last second, Luka paused and turned around.

"Come with me, Sylvia."

The young water mage jumped up, hearts in her eyes. "Syl will follow Luka-sama anywhere! But where are we going?"

Luka grinned. "You're always proclaiming yourself my future wife. Let's elope."

Sylvia blinked. Then she beamed and chased Luka out of the guild hall, leaving Natsu, Gray, and everyone else with their jaws on the floor.

"WHAT?!" Gray roared, but they were already gone.

Luka and Sylvia leapt into the back of the carriage and Ian cracked the reins. The wheels bounced wildly down the stone streets of Magnolia.

"Gonna hurl!" Luka immediately got motion sick.

They were thrown around comically in the back of the carriage every time Ian hit a bump.

The carriage ride was about four hours, and Luka was sure he was going to die.

Finally, Ian slowed it to a halt. "Welcome to my summer home, you guys!"

Sylvia and Luka poked their heads out the back of the carriage, Luka's face still green. In front of them was a huge mansion with a private beach in the back.

Luka looked at Ian. "You're rich?"

Ian blinked. "You mean you didn't know? My dad's the mayor of Magnolia."

Sylvia looked depressed. "What about Luka-sama and Syl's elopement? Is Ian a love rival?!"

"No way he is!" Luka yelped. "And that was just a cover story. I've barely turned seven!"

"Aww..." Sylvia pouted.

Luka quickly explained what Ian had seen, and how he might've found where Aquarius's key reformed.

"So, I thought your water magic would be helpful to get to the bottom of the ocean." Luka finished.

"Syl understands." Sylvia beamed. "Syl will do her best! She will always help Luka-sama!"

Luka blushed a little. "Thanks."

The three of them ran behind the house and Ian lead them to a dock with a sail boat. "I'll take you to were I dove down. You sure you don't need equipment?"

Luka shook his head. "I trust Sylvia."

"Syl loves Luka-sama!" Sylvia squealed.

Ian chuckled. "You're quite the dynamic duo."

"Ian ships Syl with Luka-sama! Ian is no longer a love rival!" Sylvia proclaimed. She and Ian high-fived while Luka facepalmed.

Ian expertly maneuvered the boat to a certain spot in the ocean while Luka hung over the edge and Sylvia held him to keep him from falling overboard.

Luka and Sylvia jumped into the cold water, and she created a giant bubble of air for each of them.

The two wizards swam into the depths. Soon the bottom of the ocean was visable.

The drop off to the ocean proper was quite a sight. Some watery light still reached down here, but beyond the shelf was pitch black nothingness.

Thousands of multicolored fish swam in schools around them. The sea floor had tons of exotic flora in all shapes and sizes.

A golden glow caught Luka's eye. A gold key was balanced precariously on the edge of the shelf. Luka pointed at it, and Sylvia nodded.

As soon as they got within twenty meters, a strong current almost sent Luka reeling away. Sylvia's magic was the only thing that stopped it.

"You're not her." An angry female voice echoed directly inside their heads.

"Aquarius?" Luka asked.

"Congratulations, you know who I am. Now leave, you damn brats!"

"My name is Luka Dragneel!" He shouted. The current died down a fraction. They swam closer.

"Dragneel?" Aquarius asked.

"I'm the son of Lucy Dragneel. Heartfilia is Mama's maiden name."

"Hoh? So that little girl finally managed to get herself a boyfriend?"

Luka and Sylvia landed on the ocean floor, a few feet from the key. The water above it shimmered and a busty bluenette with a blue tail appeared. She narrowed her eyes and put her face close to Luka's.

"Humph. You do have Lucy's eyes."

"Mermaid!" Sylvia gasped.

"Hah?" Aquarius looked at Sylvia angrily. "What'd you call me, you damn brat? Do you want me to sweep you away, hah?"

"Mama's been searching everywhere for your key. She vowed to find you!" Luka said.

"You don't think I know that, hah?" Aquarius flicked her tail, looking happy. "Even though I told her I couldn't see her again. Silly brat."

"Then make a contract with me!" Luka suggested.

"Hoh?" Aquarius looked haughty again. "Why would I want a brat like you for a master? Besides, you're not even a celestial spirit mage."

"Don't you wanna see Mama?"

"Tch." Aquarius turned up her nose, closing her eyes. She cracked one open. Luka held out his hand.

Aquarius wavered for a second. Then, with another click of her tongue, her key flew into Luka's outstretched hand. Her image disappeared.

"I look forward to working with you!" Luka grinned. Sylvia smiled, happy because he was happy.

They swam up and broke the surface. Ian fished them out of the water and gave them blankets, then directed the boat to shore.

Once on dry land, Luka and Sylvia wrung out their water-logged clothes as best they could. Luka had brought a suitcase full of spares, but the two never got a chance to change.

Their parents had sniffed them out.

"Sylvia!" Juvia shouted.

"Luka!" Natsu and Lucy yelled.

"WHAT THE HELL DO YA THINK YOU'RE DOING TO MY DAUGHTER, YOU LITTLE SHIT!" Gray screamed lovingly.

Sylvia stepped forward, but Luka held out a hand stopping her.

"We're not coming back—"

"Oi, Luka!"

"—unless Mama fights me." He finished.

"Huh?" All four adults stopped short.

"Me?" Lucy pointed at herself in surprise.

"Hey Mama," Luka said softly. "Solano-san had Aries' key before you, right?"

Lucy nodded, unsure of where this was going.

"And she got Aries' key by beating Karen Lilica in battle. Then you got Aries, Gemini, and Scorpio by defeating her in battle."

"Well, technically no—"

"In other words, fights between celestial spirit wizards are winner takes all, right?" Luka asked. "So if I happened to have a contract with a spirit and you beat me, you'd get that spirit?"

"Your contract with said spirit would need to be released, by death or incarceration or the likes. And it depends on the spirit. I wouldn't contract a spirit without their consent."

"Okay." Luka nodded. "Fight me seriously, Mama!"

Luka leapt at Lucy, lighting his fists on fire. He punched her with all his strength, making her step back. Lucy knew then that he was serious.

She pulled out her whip and got it to wrap around her son gently. Then she heaved and tossed him out into the surf.

"Why are you doing this, Luka?" She asked.

Luka didn't respond. Instead, he took something out of his pocket and dipped it in the ocean water.

Lucy felt her world tipping. That was a key, one she recognized. Her first friend.

"I'm not sure if I can actually open this gate because I'm not a celestial spirit mage, so I'm counting on you to come out on your own again! Open, gate of the Water Bearer: Aquarius!" Luka shouted.

Natsu, Gray, and Juvia all watched on in utter disbelief. There was the sound of a door bell. Lucy began to cry.

A very familiar water spirit with blue hair and a bad attitude appeared behind Luka.

She and Lucy stared at each other, both's eyes swimming with emotion. It had been years since they saw each other, after the fight with Brandish during the war against the Alvarez Empire. At that time, neither was sure they'd ever see the other again.

"Tch. You aren't a little girl anymore."

"Aquarius." Lucy said.

"Fight me, Mama!" Luka yelled a third time. He pointed Aquarius's key at Lucy.

Lucy wiped tears from her eyes and smirked. "Very well. Mama's gonna use a dirty trick she learned from Solano. Open, gate of the Scorpion: Scorpio!"

"Aquarius!" Scorpio said. "Glad to see Lucy finally found your key. We are!"

Luka had heard the story, so it came as no surprise when Scorpio and Aquarius began fawning all over each other. He leapt at Lucy again.

A few minutes later, Luka was panting with his hands on his knees, despite Lucy holding back. He fell over in exhaustion, and Lucy won.

Luka lay spread eagle on the sand. Aquarius floated over. He laughed. "I guess fire and water don't mix well, huh, Aquarius?"

"Syl disagrees!" Sylvia shouted in the background.

He lifted her key. "I release you from our contract."

"This is the shortest contract I've ever had." She snorted, and looked at Lucy.

Luka offered the gold key to her. Lucy took it. Everyone grinned, watching her cry while the mermaid awkwardly attempted to comfort her.

"Welcome home, Aquarius!" Lucy wailed.