Sans is a MONSTER and not a HUMAN, so he doesn't follow HUMAN biology like a HUMAN skeleton. Same with Gaster and Papyrus. 

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"He's crazily observant for his age..." The robot felt the need to point out the most obvious thing in the world, leaning against the doorframe to the living room, as he and Sans were watching Papyrus play with Leo, keeping the child entertained, as he was currently laughing and giggling, being filled with joy, because he was currently distracted from his life problems. Sans sadly grinned, knowing that was very true. He was insanely observant, that sometimes even as a full grown adult, the skeleton felt like it was impossible to keep everything hidden from him. He sighed. That was the problem when he of all people was Leo's father - he was too much like him. "What happened anyway?"

"...what do ya mean?"

"You know what I mean, Sans." Mettaton said, staring towards the shorter monster, who was staring back towards his child playing with his little brother, rather than returning the robot's stare. "With you and Frisk? You were so happy." He commented, only for Sans to quietly growl under his breath. He didn't want to have to keep explaining it, over and over. He just... Even for a bit, he just wanted to stop thinking about it. He hated that even here, he felt like his mind wouldn't go away from the depressing situation. The more he thought about it, the more miserable he felt. 

It was probably for the best that he was here too, and that he didn't just drop Leo off to have some lighthearted enjoyment with his uncle. Because if he had been alone, he might have gone to Grillby's, ordered a drink... He hadn't drunk himself stupid in a long time, but something told him that if he had the capability, he might just try it again; he just wanted to stop thinking about it. About how much it ached, being apart from Frisk. His soul hurt, thinking about how badly he just wanted to hold her, kiss her, love her... But he couldn't. 

"i've got an idea." Sans spoke up, more towards Papyrus and Leo, so that he could change the subject, and not focus on the robot's curious stare. "how about we go to the park? ya like the park, don't ya, leo? uncle papyrus can push ya on the swing." Sans suggested, seeing the boy look eager to go outside. Leo may have been an introvert, and may have been more kept to himself, but he definitely enjoyed getting to spend time with his family. Especially outside, in the park. Compared to Luna though, he was the calmer one. 

"Yeah! Can we?" Leo asked, also looking at his uncle as if he also needed his permission. Papyrus chuckled.

"OF COURSE! THAT SOUNDS WONDERFUL!" Papyrus nodded, thinking that going outside and enjoying some time in the fresh air might not only do some good for Leo, but it might just do some good for his older brother too. Mettaton on the other hand was still looking annoyed about the fact that Sans had just ignored him and decided to change the subject, but he didn't say anything on it, thinking perhaps that was just for the best. 

On the way to the park, Papyrus and Mettaton stood either side of Leo, holding one of his hands each, seeming to lift him up into the air every now and then while he laughed. Trailing behind, Sans walked, with his hands stuffed into his pockets, a sad smile trailed along his face. Mettaton and Papyrus honestly looked like an actual family, when they were entertaining his son. They really would be good parents one day, he believed in their ability to take care of a child. Especially when they made his son so happy - something he wasn't that capable of doing at the moment, in his current state. He wanted to make him happy, but was he doing that good of a job at it? He didn't feel like it...

Arriving at the park, the first thing that Leo went for was the swings, because he wanted to be pushed by his uncle Papyrus. Mettaton also stood close to them, smiling at the fact that Leo looked pretty chirpy, compared to the way he had been when he and Sans had just been sitting in Gaster's  house. He was actually laughing, like he was having a blast, enjoying his childhood, like a 6 year old should. Sans watched from a short distance, just feeling like he needed a moment to himself. At least he could get that, with those two keeping the small boy entertained. 

He wondered what Frisk was up to right now... Was she working? Or was she spending time with Luna? Kindergarten should have ended by now, so he was hoping that she had gone to pick Luna up. He would have been pretty pissed if she had just sent someone else to do it, like she didn't have the time to fetch her own daughter from school. He exhaled. He knew he was pretty pent up on the whole fact that she had missed Leo and Luna's play, but he couldn't get the image of Leo's disappointed face out of his mind. Seeing his son so distraught, and because of his own mother, it had really bothered him. 

No, Frisk wasn't a bad mom. No, Frisk wasn't a terrible person. It was just... Sans couldn't handle it. He couldn't handle her seeming more attached to the Mafia than anything else. He couldn't stand the thought of all her time getting taken up, so that not only would he not get to spend hardly any time with her anymore, but the same with their kids. Their happiness was extremely important, and he didn't want them to be unhappy. He wanted to think of them constantly, wanting the best for them.

Glancing around the park, it wasn't hard to notice other parents spending time with their kids, or watching them from a park bench, while they were having fun with their friends. For a second, Sans' pupils had trailed past the closest park bench, not really caring about who was sitting upon it, until he did a double take. Wait, the person who was sitting there looked oddly familiar. Trailing his pupils back to the bench, he rose his brows a tiny bit when he recognised that green hair. Kai? What was she doing here? Not to mention, she looked like she was having a pretty cloudy day herself. Huh...

"hey, uh... i'll be back in a sec." Sans told the others, before he decided to walk over to the bench, keeping his hands stuffed into his pockets. When walking over, Kai hadn't really noticed him; she had been more busy sadly staring down at the ground, about whatever she seemed so gloomy about. "is it just not a good day for anyone today?" Sans weakly joked, noticing the girl jump a little, as she must have been lost in thought. Her eyes ripped away from the ground, to stare up at the skeletons face. 

"O-Oh, Sans." Kai weakly smiled. "Hey..."

"what are ya doin' here?" 

"Um... Not much." Kai shrugged. "How come you're not with Frisk?" She questioned, only to notice a strange look flash across the skeleton's face. Her comment caused Sans to glance elsewhere, once again finding himself hating that people had to keep asking that question. Kai watched him with confusion, only to be in shock when he told her the truth - that he and Frisk were no longer together. "W-Wait, what?! Why not?" Kai blurted. Frisk had been so crazily in love with him, that he was all she talked about a lot of the time when they were at work. Not that she minded, especially when Frisk talked about the engagement - it was adorable. She was happy for her. "Well, come on, take a seat. You look like you could do with some ranting." Kai offered, patting the empty space beside her on the bench.

"w-well... i shouldn't..." 

"I insist." Kai replied. At first, she had been scared of this skeleton when meeting him for the first time; he gave off such an intimidating vibe. But after awhile, she had gotten used to the way he acted. It was easy to do that when he used to come into the cafe quite often with Frisk. He visited often during the time that he had been soul pregnant with Leo and Luna, and he would order food from their cafe a lot. So, in a weird way, he had become her friend. And she was happy to be his friend, seeing as he was Frisk's... Well... He was Frisk's fiance. For a short time, the skeleton hesitated, only to exhale and take a pew beside the human girl with freckles. 

"it's kinda a long story..." Not one that he could explain in full detail; he couldn't tell Kai about the Mafia. That would be crazy. "we just... couldn't come to an agreement on certain things i guess..." He shrugged, not really knowing how else to put it, while the green haired girl stared at him, being kind enough to actually listen to his thoughts. No, she wasn't the only one to offer, but right now, Papyrus and Mettaton were keeping Leo entertained, and he didn't want to bombard them with shit while they were doing that. 

"I've been worried about her... She stopped coming into work as much... She even let go of her job at the cafe today." Kai admitted with a frown, seeing clear shock across the skeleton's face. "She even looks like she hasn't slept." She added. "...I guess that's why?" Kai wondered, seeing Sans look conflicted about what he wanted to say or how he wanted to react. Frisk quit her job at the cafe, for the Mafia? Half of him wanted to have an outburst of outrage, questioning why she would even do that. He remembered when she had been so excited for that job, to get hired. And she just quit? To be the Mafia boss? Was she insane? 

"...i guess..." Sans mumbled. "i guess that's why ya were bummed out then?" 

"A bit. Working with Frisk is nice - she's a kind person." Kai sadly smiled. "She definitely made working at the cafe more enjoyable, especially with all the shitty customers we would get... She would be so brave, and cool, dealing with them as if it didn't phase her." Kai went on, staring forward with thought as she talked about Frisk with such positivity. Sans knew why Frisk was so brave: she had been through a lot. So, ordinary people in a cafe probably wouldn't phase her anymore. "I'm definitely going to miss working with her, if she doesn't decide to come back."

"...hopefully she will." Sans answered. 

"Hopefully... I hope you work things out with her too." Kai beamed, earning a tiny grin from Sans in return at her comment. "You two are sweet together, so it would be a shame..." Kai said honestly, knowing she herself would be disappointed if her friend and her fiance weren't able to work it out. Her friend's happiness meant a lot to her. 

"thanks... i hope so too..." 

He really, really did.