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 had expected himself to hate this with a burning passion, and part of him still truthfully did. The 6 year old didn't really see himself as someone who liked to act, but his sister on the other hand, enjoyed it with all her heart, especially with how much she liked to play pretend with her dolls. She probably had a wider imagination than him, even as a 6 year old he didn't feel like he was someone who was all that imaginative. Despite that, he was actually finding himself enjoying practicing this little play that they were doing. His older sister's part was an angel, while he was for some reason given a bigger part. Of course, because they were little kids,  the play wasn't all that complicated with loads and loads of lines, but he still had more time to be "on stage" than his sister was. 

Leo had never asked for the larger part, it was just for some reason handed to him by his teacher, who apparently had faith in the 6 year old that he would be good at it. In this whole Christmas play about baby Jesus, he was given the part of... Of... What was his name? Joe? Joey? Or was it something like... It was something weird, like Joseph. Some of the other kids in his class had wanted the part, and they  were disappointed when it wasn't given to them. It was amazing how much energy little kids could put into throwing a tantrum, and it was even more amazing how adults still managed to not give them exactly what they wanted despite their efforts because life didn't work that way. 

Despite being a child who was already an introvert, Leo was actually rather excited for performing this play in front of his two parents, especially when they motivated him to put the effort into this whole thing, when a part of him didn't particularly enjoy it. Perhaps it would be worth it once it was all over, and he saw how proud his mommy and daddy would be of him. After all, like Luna, he looked up to his two parents, and so their opinions meant everything to him. Which was why he hoped he was able to do his part well, and he didn't end up looking like an idiot. It was funny how most kids probably didn't care about looking silly, but he did. 

The brother and the sister agreed that they wouldn't talk about the play properly until they actually performed it. As Luna said it, they weren't getting any spoilers! (Or "no spoilies", is how she actually phrased it, trying to seem cute). Of course that made it extremely cute, because Luna excelled at being adorable. Normally, parents only pretended to be excited for what their children were doing, just because they wanted to make their kids happy and that was about it. But Sans and Frisk were actually genuinely looking forward to their play. 

Sans and Frisk cherished the moments they got to spend with their children. Any moment they were able to make them feel proud of themselves, they'd gladly go for it, because their children's happiness made them happy too. They liked being involved in Leo and Luna's lives because they were growing up fast, and it would only be a matter of time before they started getting even older, and older. It was crazy to the both of them that they were already 6 years old, despite it feeling like just yesterday they were born. Sans could never, ever forget that whole experience, just because of how frightening it had been for him. Those whole 7 months had probably been the most scared he had ever been, but after it was all over, it felt so, so worth it.

-Flashback- 

Getting over the fact that he was actually a dad now wasn't exactly easy, not that he was complaining. In fact, the skeleton was so overjoyed, he was finding it hard not to cry, when he found himself alone with Leo resting on one of his arms, and he was using the other to support Luna. He just wanted a second for it to be just them and him, and Frisk seemed to have no issue with that. She actually smiled widely at him before she left, as she must have found it so cute just how attached to them he already was, after only a couple of hours of them being in this world. 

"sorry... daddy isn't usually this much of a crybaby, heh." Sans softly chuckled, as he sniffled, and tried to use one of his shoulders to wipe one of his eyesockets due to the fact that tears wouldn't stop leaking from them. But it's not like it did a whole lot, as he only continued to cry, even after he had tried to clean his face a bit. But these were happy tears. Happy tears, as he stared over his two children. Luna was staring back at him with those two black sockets of hers, while Leo on the other hand had fallen fast asleep, looking quite peaceful as he slept. The father hoped he was having nice dreams. 

"listen... i know ya two probably can't quite understand me just yet... but... i'll try my best to be a good dad to ya both, okay?" Sans whispered to them, especially trying to keep his voice quiet due to the fact that Leo was sleeping, but he hoped he was still listening despite the fact that he was currently far away in dreamland. "i never expected to be a dad. but i'm glad to have ya both. never forget that, daddy already loves ya, so much." He continued to whisper, before lightly kissing both of them on their heads. Luna ended up making soft gurgling baby sounds in response which only made Sans smile, while Leo continued to sleep peacefully. 

This was just crazy, how he never knew this could happen for him. Never in a million years, did he think he would ever have kids. He always assumed that after what happened with Blair, he would probably only die alone, and at the time, when he had been a complete miserable, grumpy skeleton, he had been completely fine with that. He had been okay with the concept of dying alone, of leaving nothing behind, because he just didn't care. He hadn't cared about anything for a long time, and he didn't think he was capable of caring again. Then Frisk came along, and she changed his entire world. She gave him things he thought he would never be able to have, that he didn't think he cared about anymore. But these two little ones, he loved them more than life itself, just like Frisk. He loved his family. 

"though... don't expect me not to embarrass ya on a couple of occasions. maybe it'll finally be my chance to whip out some dad jokes." Sans joked, chuckling a little once more, before he fell quiet, and he just stared at these two perfect beings with so much fatherly love. Normally, when Frisk was in the room, she decided to start humming to them gently, to give them something to listen to and it was part of the reason why Leo had drifted off to sleep. He didn't really know if he had a nice voice like Frisk to be able to hum anything for them but... He found himself doing it anyway, just humming some small tune, and he could have sworn Luna began to smile as he did this, until she slowly began to drift off to sleep just like her little brother. 

They were just so perfect. 

-End of flashback-

Which was why both of them deserved all the love and attention in the world. Even if Sans would be unsure of what he would do as a different career path, he would gladly throw away his job if he had to for his children. It's why he didn't work as often as he used to, and why he got Frisk to quit the Mafia, because he didn't like how much of their lives it would take up. Sans always put his children before himself, always. And he would gladly do that at any point in a heartbeat, (even if he didn't have a heart). 

His family was his world.