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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Juggling two jobs while having to take care of a family was admittedly somewhat difficult, however, it wasn't something that Frisk felt like she couldn't handle. For the first year of Leo and Luna being around, she had just mostly decided to stick with her cafe, waitress job, serving people in there while Sans had gone back to doing his Mafia thing, as it was really the only thing he knew. He had tried countless times to try and get a different job, heck, he even tried to work in the cafe once with her as a waiter, but some of the customers just really pissed him off that he couldn't take it. And it wasn't like a waiter was supposed to punch someone across the mouth, (which he might have done and got fired for). So, he went back to the whole Mafia game. 

And Frisk... She had been getting more involved herself, but she also liked to have this normal, more calmer job, as it reminded her more of reality, and she never wanted to get to the point where she would feel like she had lost complete grip of herself in all the craziness that was being in the Mafia. She just liked to be surrounded by more ordinary people now and again, than the crazy murderers she had to deal with on her missions. 

Even after six years of working here, another person that still remained at this cafe was none other than Kai. She still pretty much looked the same, except that she had chosen to grow her green hair a little longer, so it went half way down her back. She actually looked extremely pretty with long, green hair. And she was just as sweet as always. She liked to have a friend like Kai who was just an ordinary person, but of course she would never trade in any of the friends that she had made from being in the Mafia. She loved all of those guys to death. 

"You look tired." Kai commented towards Frisk with a small smile. "I'm guessing that's the result of parenthood." She giggled, causing Frisk to chuckle a little herself. It was partly that as well as the fact that she had two jobs. Sometimes it was pretty difficult to get someone like Luna to go to sleep at an early hour. Leo on the other hand was another story, he could lie down in bed and fall asleep just like that. But Luna was a bubbly, bouncy girl who was so full of energy, you could be up for hours trying to get her to sleep. She wouldn't change it though. 

"Yeah... My daughter is just super energetic, even at night time, so it takes forever to get her to sleep." Frisk chuckled, seeing Kai smile at that. She could still remember the early days of parenthood like it was yesterday, and how scary and new it felt. Heck, six years later, it still felt like a scary, chaotic, but exciting experience. Even now, Frisk felt like she was still learning new things about trying to be a good mom, and making sure her kids were satisfied with the lives they were having. She wanted to make sure though that they never knew about what her and their father did, the fact that they were part of the Mafia. Her and Sans had agreed from the beginning that they would never allow either of them to get involved in that. 

-Flashback- 

"They're so precious..." Frisk had whispered, standing next to Sans, as he had an arm across her shoulders, holding her close. She was resting her head on his shoulder, just staring down at the two sleeping babies with a wide smile across her lips. It had been already a few days at this point since they had been brought out into the world, and even now Frisk was so emotional over the fact that they were actually here. Even Sans was still pretty emotional here and there over the fact they had two children together. They were actually parents. 

"...i'm glad that i chose to go through with it. that i told ya." Sans smiled. All of this could have almost never happened, if he had chosen to be a selfish idiot and if he had just gone through with getting rid of what he had thought at the time would be only one child, because he had felt too terrified to go through a monster pregnancy. It wasn't until he had a complete breakdown in front of Frisk that she found out the truth, and since then they went through the whole thing together. "it is tough... and i am still really scared..." Sans admitted quietly. "but i'm sure, together, we can look after them in the best way we can." 

"Yeah..." Frisk agreed with a nod, causing the two of them to fall silent for only a minute, still watching Leo and Luna soundly sleep. Sans was thinking for a moment about something else he wanted to say, and he thought it was best to bring it up, wanting to get Frisk's opinion about the whole thing. Though, he had a feeling she would agree.

"ya know, i think we should... try our best to keep them away from the whole mafia thing." Sans said. "while i don't regret my dad getting me involved in such a thing, it's still pretty dangerous... and i just couldn't stand involvin' them in such things." The skeleton went on. He knew that he could just quit his job, but then if he did do that, what then? He probably wouldn't be able to find work anywhere else. He could probably try and work at that cafe with Frisk, but something told him it probably wouldn't go very well... He just knew that if anything ever happened to Luna or Leo, he would be in pieces, and enough bad things had happened in his life. He didn't know if he could ever take losing a child, as he was already so damn attached to them. Losing them might just break him, and cause him to crumble to pieces. 

"I agree." Frisk nodded, not even hesitating to answer his suggestion. She too knew that if anything ever happened to their babies, she didn't know what she would do. She didn't know if she would be able to handle such heartbreak. Seeing her parents die before her eyes had been one thing; dealing with it had been hard enough. But losing their children? She wouldn't be surprised if she died from a broken heart. "I want their lives to be as normal as possible..." She murmured. The way it should be. Like making friends, going to school, maybe even going to parties... Just growing up stuff. Frisk didn't regret the way she grew up, but she wouldn't encourage it to her children. 

"glad we're on the same page." Sans beamed, and with that being said, the two fell quiet again as they stared over their children, really not knowing what else to do with their time other than this. They really were going to be protective parents. 

-Flashback over- 

"She sounds so cute... You should bring her here to eat more often. It would be nice to see her again." Kai said, causing Frisk to say she'd think about it. Cause sometimes she had brought her son and daughter here to eat, sometimes with Sans and sometimes without him when he was at work. Frisk was just glad that the Mafia didn't end up sucking up all of her boyfriend's time, and that he was still able to make time for her and the children. Because when Sans had decided he wanted to go back into being a part of the Mafia, she said that it was okay, just as long as their children weren't involved, and he still made time for them. 

She was proud of him for continuing to be the best dad he could be.