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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Listening to their daughter ramble on about how nice Santa was, and how she hoped she had been good enough to get the gifts that she wanted for Christmas, had to be one of the most precious things in the world. For Sans, when standing in that line, it had felt like the time had been dragging on forever, considering it was moving so insanely slow. They probably had to have been standing in that line for at least an hour or two, before they finally got to the front. For the oldest skeleton, it definitely felt exhausting, but it made him happy to see his two children so excited to meet Santa, so just to see their smiles definitely made it pretty worth it. Sans knew he had changed a lot, since meeting Frisk. He cared a lot more, and that wasn't at all a bad thing. He was glad that this was how his life turned out, he wouldn't have asked for his life to go any other way. 

"I hope Santa isn't too mad at me this year..." Leo stated, as he was holding one of Sans' hands, while Luna was holding the other. They were on their way home from the mall, and Frisk was trailing behind with Chara at her side - considering they had found each other in the mall. Frisk didn't have any hands free to hold her children's hands, considering she was carrying bags of shopping, (which she claimed was for other people when asked by the children, as she said that Santa would be bringing their gifts), so it was good that they had absolutely no issue with clinging to their father's hands as they walked along. 

"why do ya think he'd be mad with ya?" Sans questioned, seeing the slightly worried look on the 6 year old's face. Yeah, there had been a lot of ups and downs this year. He knew, if anything, this year hadn't been amazing for Leo, with the way things had been going in kindergarten, with feeling like he was being rejected by people, and the fact that he broke down into tears during his play... He threw tantrums when he didn't want to go to school, and his attitude hadn't always been the best, but... He was just learning. He was only young. 

"There were many times I was bad... I yelled a lot, and got angry a lot..." Leo stated, staring down towards the ground with those two sad eyes of his. Sans weakly smiled, wanting to dispel Leo's worries. He knew he wasn't disappointed in him for how things had been this year. It was understandable to be upset when you felt like everyone hated you, and it was understandable for him to get angry when Frisk missed his Christmas performance. The father lightly squeezed Leo's hand to get his attention, and it did just the trick as the small boy looked up at his dad with a frown.

"Santa won't be angry with you." Chara spoke up from behind them, while Frisk smiled and nodded in agreement. "He looked very happy to see you when you got to the front of the line." After all, Chara was just able to witness that when her and Frisk had been coming back from shopping, and finally Sans, Leo and Luna were at the front of the line to see Santa. The excitement for the twins had been extremely obvious upon their faces, while their eyes gleamed with wonder and curiosity. It had been too cute. Sans and Frisk were lucky to have kids like Leo and Luna.

"Yeah, Chara is right. I'm sure Santa is very proud of you this year, for trying your best." Frisk said, sending a knowing look towards Sans who looked over his shoulder at her with a warm smile. The parents knew they were proud of both their children, even if they struggled sometimes. If anything, Sans and Frisk were extremely proud of Leo for finally being able to make a friend - Kyle. Sometimes, even at a young age, it was quality over quantity that mattered. Kids may have felt like they needed a ton of friends, but that was not true. It was always the people that decided to stick around for you in the long term that mattered, not those that were just there for the label of being a friend. 

"...I hope so." Leo smiled, definitely feeling a little better when his mom decided to say that, especially because she was one of the people who he had gotten angry towards this year, and he had felt very bad for doing so, even though she didn't seem to care that he was mad at her. She had just seemed relieved to see him when he decided to come home with Sans that one day. This year had been pretty tough on Leo, but the parents and even his sister was hoping that things would be much better for him next year. 

When arriving home, Leo and Luna went to take off their winter boots, scarves, gloves and hats, alongside their coats, just so they could rush into the living room and play. Chara couldn't help but smile, as she watched Sans sort out the mess they had left, with one boot just being carelessly tossed to the side, not even sitting beside the other boot. But they were kids, so it wasn't like they would be very tidy anyway. It was just nice, seeing Sans acting like a nice dad, even if he had already been a dad for 6 years. Sometimes it was hard to believe that that asshole skeleton she knew from several years back, that she absolutely hated, was exactly the same person that was right in front of her, dealing with the mess of his children. Because he was so crazily caring now, compared to back then. 

"Do you want me to help you with that, Frisk?" Chara wondered, as Frisk was taking her bags of shopping upstairs, and Frisk happily accepted the offer, giving one of the bags to Chara once she joined her upon the stairs. For the moment, the girl dressed in green decided to stay silent, as she was following after Frisk, and thinking about what she had said earlier to her in the mall, about her and Asriel. How in the world was Frisk planning to help her and Asriel get together? Not knowing what Frisk would be up to at their party did admittedly make the crushing girl a little bit nervous, because the last thing that she wanted to happen was her looking like a complete klutz in front of her long term best friend. 

She wanted the party to go well, and if it all went to shit just because Asriel ended up finding out about her feelings, then... "Stop stressing." She heard a voice say, causing her to snap out of it, and she saw Frisk smiling at her as they were in her and Sans' room. Frisk was taking the bag from her grasp, to slide it underneath her and Sans' bed, as a hiding spot, so that the kids would never find them. After all, Leo and Luna knew they were never allowed under their parents' bed, so it's not like they would ever go looking. "It's odd to see you so worried about such a thing. I thought you were the relationship expert." Frisk teased, lightly nudging Chara who grumbled and pouted as she crossed her arms.

"Yeah, well... I may have helped you and Sans but..." Chara said quietly, until she trailed off. She had never really felt this way about anyone before, and the fact that it was someone who was actually a huge fucking deal to her, who she knew she would hate to be separated from, if it all went wrong, then... She wouldn't be sure how to react. She wasn't sure she could handle having her heart broken in such a way. She wasn't sure she could handle not only losing her long time best friend, but the guy who she felt liked she loved, the only guy she felt like she could really love, and she knew thinking that probably sounded over dramatic but... How could she not think that about him? He was kind, loyal, sweet, funny... Just the best person she could ever ask for. 

Ever since Chara had been practically rescued by his family, considering she had nowhere else to go, all he had ever done since then was be there for her, laugh with her, cry with her, and even help look after her. Even when they were growing up together, and he wasn't that much older than her, he protected her, despite her never having asked him to. But despite all his kind actions, Chara was fearful that he loved her in a whole different way. Not the romance kind of love. 

But in a 'family' kind of way.