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

--------------------------------

Once again, it was one of those days where Leo and Luna had to go to kindergarten, considering it was a week day. Luna really enjoyed kindergarten, because she had so much fun making new kinds of friends. Her brother, Leo, was quieter than her though, so he did have a harder time making friends, (it was hardly questionable who he took after). He did have a few friends though; he just wasn't like Luna where she had made friends with probably almost every person in the classroom. 

While Luna was on one side of the classroom, playing with a couple of puppets with some of her friends, Leo was minding his own business, deciding to build a tower out of colourful building blocks. In fact, he was actually quite proud of what he was building, as he had managed to make it quite tall. Of course, with other kids in the classroom, that wasn't going to last very long. Another boy who was probably a little younger than he was had decided to come over, and went to knock his tower over. "Hey!" Leo yelled at him in irritation. "Why did you do that for?!" The kid didn't answer though, as he was too busy picking up some of the blocks that used to belong to the tower that Leo was building.

"Those are mine! I was using them!" Leo argued, going to try and snatch them from the boy's hand, but the boy just started to whine and hug the blocks to his chest as he wanted them. The six year old grew increasingly frustrated, wanting his blocks back. While the two were arguing over who the blocks belonged to, it was only inevitable that a teacher was going to notice what was going on. Leo tried to point out that he was using the blocks, but like any teacher would really do, she mentioned how they had to share. This only upset Leo more. 

"NO, THEY'RE MINE!" Leo screamed, causing his two bright blue eyes to flare a little brighter alongside his harsh tone. Because while he was partly human, he and Luna both possessed magical abilities that they could use - just like their father. Though, the moment that Leo had gotten so furious, everyone in the room seemed to draw away from him, and he noticed it. He glanced around the room, seeing everyone looking at him with fear, well, apart from his sister. She was staring at him with a look of surprise across her face. Saddened surprise. 

After such an incident, of course the teacher was going to choose to call Leo's parents to talk to them about what happened. Frisk wasn't currently home, so Sans had to end up being the one who had to pick Leo up after a little conversation with the teacher. He was told on what happened, and if it had really ever happened before. Leo was more so the quiet type, so him yelling at people over things rarely ever happened. Especially because him and his sister got along so well. But Sans just decided to take him home for the day to make sure he was okay. 

"what was that all about?" Sans asked his son. All Leo did was shrug, because he didn't really know what to say. He was only 6 years old after all. It wasn't like he was a grown up teenager who completely understood the reasoning behind his actions. Sans sighed. "ya shouldn't ever use your magic against someone in your class... it'll get ya into a lot of trouble." That, and the skeleton didn't really like the idea of Leo taking after him, following in his footsteps. When he heard comments that Leo was "just like his dad", he wasn't sure if it was something to be proud of. The last thing he wanted was for Leo to be an antisocial, ill-tempered mess like he could be half the time. 

"I know... But I don't know how to contwol it. It just happened." Leo murmured, sounding worried that his dad was angry with him. "I didn't mean to do it." He added. Especially after seeing the way everyone looked at him. They were all scared of him, even for a 6 year old he had noticed that. The only person who probably hadn't been frightened by him was his sister, but she still looked upset. 

"i know... i do want to teach ya about magic someday, but i wanted to wait until ya were a bit older to really understand it. you're not in trouble, leo. i'm not angry. but just... try to avoid doin' that again in the future." Sans explained. After that, the two of them fell quiet on the rest of the walk home. It was clear Leo would be thinking about what happened for awhile. Just like how he sometimes overthought things a little more than they needed to be thought about. Sans did wish that Frisk could have been able to also be there to reassure Leo, but right now she was currently on a mission, so no one was able to get in contact with her. 

Admittedly, Sans was starting to dislike how involved Frisk was with the Mafia, only because he had always hated the idea of her becoming someone different to who she originally was. True, she was a lot more of a confident person nowadays, and she didn't scare away as easily but... Sometimes he wasn't sure if that was a good thing. He remembered the days when Frisk would give him shit for being so unfazed by the idea of murder, and while at the time it had been somewhat annoying, it just showed she was more human. She was still in contact with her emotions. But ever since she had been involved in that mission that Undyne had set her on to go after her parent's murderers, he definitely had noticed a difference in her over the years. 

What was slightly more infuriating was how Frisk would likely give him shit for missing something that was important to their kids, but where was she today? 

-Flashback- 

Today was Luna and Leo's first day at kindergarten. It had been pretty difficult to say goodbye to the two, as Frisk wasn't used to them being looked after by some stranger. Well, she knew it would be a teacher, but it wasn't like she knew the teacher personally. But still, she knew that she had to let them go for a few hours, and it had been weirdly emotional to do so. What angered her at the time was that Sans wasn't currently there, on their kid's first day. She knew she was going to have something to say to him once he got home.

"Sans the skeleton!" Frisk placed her hands on her hips, glaring at him the moment he had gotten through the door. He looked pretty tired after a long day at work, but that didn't mean she wasn't going to continue on her tirade about the fact that he had been at work while their children were starting their earliest form of education. "Where were you today?! It was Luna and Leo's first day, and you weren't there!" Frisk complained at him. "I'd like an answer from you. And not some kind of a rubbish excuse." 

"i didn't expect to get so held up." Sans said, loosening the tie from around his neck with a sigh. "believe me, i'm just as annoyed with myself for not bein' there as ya are at me. i wanted to be there." Sans told her truthfully. Because he really did. He felt guilty that he had gotten held back due to work and he hadn't been able to get back in time in order to see Leo and Luna go into kindergarten for the first time. After all, he wanted nothing more than to be a good dad to them, and he didn't want to be a disappointment to them. "i'm sorry..." 

Frisk stared back at her boyfriend, while her face relaxed a little. Seeing how upset Sans appeared, she did believe that he did really want to be there. She sighed, going to slowly walk over to him, and she went to give him a hug. "I don't think it's me you should be apologising to..." Frisk murmured beside his skull. "Just please, try not to let work get in the way of our children... I don't want to see their disappointed faces again because you weren't there." She whispered, while Sans slowly wrapped his arms around her in return.

"i know, i'm sorry..." 

-Flashback over-

He had meant that apology with all of his soul. Now, he knew that they couldn't have possibly predicted that Leo was going to need to be picked up earlier today, but... It was just the fact that Frisk was so involved now. He really wasn't liking it.

At all.