In this, the avengers won the first fight against Thanos in infinity war, so Tony and Nat are alive, and Steve is still around.

Steve: Remember how Peggy dumped Steve's blood into the ocean? Well, as it turned out, that was only a little bit of what was really out there. Hydra spies had managed to get their hands on Steve's blood and genetics . However, it was forgotten about, as hydra was more focused on the Winter Solider project at the time. However, around 2018, a Hydra scientist discovered it again, and used the genetics of Steve to create another super Solider. With the help of an egg donor, the child was created. (I know this is probably very far fetched and scientifically impossible but I'm trying my best here.)
Fast forward a few months, Steve, Bucky, Sam, Nat, and Wanda attack the hydra base the child is held in. In a rush for his life, the scientist abandons the child. "I'm going to go in and see if we missed anyone." Steve says. As he searches the complex, he hears a noise, a noise that should not be heard in a Hydra base. A baby cry. Steve was furious that Hydra would do things to innocent babies! He ran into the room he heard the cry coming from, and saw you lying on a table in the corner. "Hey, hey, it's ok little guy....er...little girl." Steve said when he realized you were a female. "Let's get you out of here."
When Steve came back, the rest of the crew was surprised to see him carry a baby. "Um, Steve? Where did you get that baby from?" Sam asked. "She was in one of the labs." Steve explained. "What? Hydra was experimenting on babies?!" Bucky said, angry. He already hated hydra for what they did to him, this didn't help at all. "I don't know. We should get her to the base so Banner can check her and make sure she is healthy."  Steve said as they all walked on to the quinjet waiting for them.
Once they got back, they explained to the rest what had happened, and banner did some check ups on her. When he came back, his face was in shock. "What happened? Is she ok?" Steve asked. "Yeah, that's the thing, she's perfectly healthy, it doesn't seem like Hydra ever experimented on her." Bruce said. Steve was confused. "But...why would they raise a child there?" "Well..I did a blood test on her...she...she has the super solider serum in her blood. And it wasn't given to her....she was born with it." Steve's heart almost stoped. If she was born with it, did that mean... "Steve, I don't know how, but Hydra somehow got ahold of your genetics...because the child is genetically yours. She's your daughter, Steve." Steve almost fell down at the news, but fortunately Nat and Bucky were right behind him and made him stand upright. Tony laughed. "Well, congrats Rogers. It's a girl!" He joked. Steve gulped. "May I see her? Alone." He asked. Bruce nodded and led him into the room where you laid, now cradled in a blanket. Steve went over to you and picked you up. You opened your eyes, revealing bright blue eyes identical to Steve. Steve smiled. "Hey sweetheart, I'm your daddy."

Tony: You are the twin sister of Morgan. Let's just say tony almost fainted when he found out there was two babies in Pepper's womb.

Bucky: Bucky found you when he was driving one night. He hit the breaks when he saw a box in the middle of the road. He ran out to see what was in the box.
He gasped when saw a baby in the box, now thankful he had stoped. He felt both sympathy for you, and also anger towards your parents. "Who could leave a doll like you in the middle of the street like this? How about I take you home with me? Would you like that?" Bucky said as you cooed and smiled.

Natasha: After they won the battle against Thanos, Natasha started feeling a bit empty for some reason. She didn't know why. She had lots of friends, who she considers family. But after Tony and Pepper welcomed Morgan into their family, Nat felt it even harder. Clint suggested it may be that she wanted a child with out realizing it. Thinking it over, she realized he was right. Obviously, she was unable to have a child on her own, so she decided to look into adoption. Tony helped her with all the formalities. Soon, she was at the adoption agency, waiting for her child to arrive. A woman walked in, caring a small bundle. "Congratulations, Ms.Romanoff,  here is your daughter." Natasha smiled widely as she stretched her arms out for you. As she held you, Natasha felt the happiest she had ever been in her whole life.

Thor: Thor was helping to rebuild Asgard on earth when he saw you laying on the ground, next to a bush. Valkyrie explained that your parents had been part of the group killed by Thanos, and somehow you had were able to get away with the others. When Thor picked you up, he smiled. "I will take care of you little one. I will make sure nothing happens to you. You are my second chance."

T'Challa- You are the firstborn of T'Challa and Nakia. The day you were born was the happiest Wakanda had seen in a long time. Many people celebrated your birth, partying in the streets and setting off fireworks. Your Aunt Shuri joked that you were going be the life of the party when you grew up, because your birth caused a wave of parties.

Peter Quill- The guardians were on earth after delivering Dr.Strange, Tony Stark, and Peter Parker back after Thanos's defeat. It had been a long time since he had been on earth. But he was not thinking about being on his birth planet. He was thinking about Gamora. His beloved Gamora who had been killed by her own father. He missed her so much. As he mourned, he heard a noise, coming from the side of the road. He went over to investigate, and was surprised to see a baby in a box. "Hey sweetie, what are you doing here?" He said as he picks you up. He knew that your parents had abandoned you. "Say, why don't you come with me? You can come have some fun adventures with us." Peter says. You giggle in response.