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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"I don't want to go back! Everyone hates me!" Of all the ways the two parents could have imagined their mornings going, this was definitely not the greatest way. It didn't make it any easier when Frisk had to go to work, and was probably going to be late. Sans was concerned that they would end up being late to taking their two children into kindergarten, because Leo was currently throwing a tantrum in his bedroom. It was very clear at the age of 6 years old already who he managed to get his temper from. 

"leo, of course they don't hate ya... they're just young like ya, and they don't quite understand. just come downstairs." Sans called up the stairs, hoping that his son would actually go and do that, that he would actually listen. But about another thirty seconds passed, and Leo didn't decide to come downstairs. He exhaled. He loved his children to bits, but being a parent could definitely be quite stressful sometimes. Especially when some of the time it felt like there was a mini version of himself in the house. A lot of the time Leo was quiet, but when he got upset, there was no doubt that he had a bit of a temper. 

"He's just upset." Luna spoke up sadly. She was already downstairs and ready with her two parents who were waiting expectantly for their son. She was trying to stand up for her brother in the worry that their mommy and daddy were angry with him, but they weren't. It was a tad frustrating, but they wouldn't scowl at him for what was going on. "People act like he's scawy. But he's not scawy, is he, mommy?" Luna asked Frisk, who only shook her head. 

"No, he's not." Frisk sadly smiled towards her daughter. "He's your amazing little brother. And you're a good sister for wanting to stick up for him." She told Luna, who looked touched by her mother's words and a small smile slipped across her lips. Sans sighed, and turned to Frisk, saying that if she wanted to get to work, she could. He didn't mind so much, seeing as it was her waitress job she had to get to. If it had been to do with the Mafia, he might have just told her it wouldn't really matter that much if she was late. 

"Are you sure?" Frisk asked, only for Sans to nod. "If you say so... Call me if you need anything though." Frisk said, going to kiss the skeleton on the cheekbone, and then she kissed Luna on top of her head to say goodbye. "Leo, I've got to go to work! So, if you don't want to miss a kiss goodbye, I would suggest coming downstairs!" Frisk tried, hoping mentally that it might convince her son to come down. But nothing. Really knowing she had to go, she apologetically went out of the door, leaving Sans and Luna waiting expectantly at the bottom of the stairs for the younger brother. 

"Will he not come to school today, daddy?" Luna questioned sadly, looking towards Sans who knew he should just go upstairs and calmly talk to him. Yelling up the stairs wasn't exactly doing a whole lot. The reason he hadn't wanted to directly confront him was just because he didn't want to upset their son even more, and cause him to get super distressed. But he might not really have much of a choice. Luna simply nodded when Sans told her to stay where she was so that he could go upstairs to try and get Leo to come down, and with that he started to make his way up the stairs. 

"leo?" Sans spoke, lightly pushing the door open to his and Luna's room, and he found the upset 6 year old boy curled up into a ball on top of his bed. He was looking frustrated and upset, as he was hugging his knees close to his chest. He even seemed to avoid Sans' eye when he came into the room and he slowly walked over. Slowly, the skeleton went to sit down at the end of his son's bed, still not saying anything for a moment, while Leo continued to glare down towards his covers. "ya need to go to kindergarten, leo." 

"No!" Leo looked up at Sans as he yelled; his voice was shaking. "I don't want to go... I don't like it. Everyone hates me." Leo murmured, causing Sans to frown at his son who had gone back to staring down at his bed covers. The father understood that it was probably difficult for him to be seen as "different". However, Leo was anything but scary. He wasn't weird looking just because he didn't have the same eyes as him or Luna. He was his son, and no matter what he was perfect to him and Frisk. 

"they don't hate ya. they just... don't really understand." Sans tried, while Leo continued to sadly look away. "i know it's hard. but if ya keep wantin' to run away, it will be difficult to gain friends. and ya don't wanna be like your old man, because that's what i used to do. i just used to run, from anythin', and everythin'. but if i kept always doin' that, i don't think i would have ever gotten with your mother." Sans said, smiling softly as Leo slowly looked back up at him, still frowning a little, but he seemed like he was beginning to come around. "it may take 'em time, but just try to get people to warm up to ya. be nice, share, and just try to be patient. your mom had to do that a lot with me." Sans joked, seeing Leo beginning to smile a little at what he said. "...i would love to talk to ya some more, but if we don't leave soon, we really are goin' to be late. so, are ya gonna come downstairs?" 

"...Yeah." Leo sighed, finally giving in. Admittedly, he still did really feel like he didn't want to go, but listening to his dad's words did cheer him up a little. He liked hearing the way his dad talked about their mom, it was clear how he cared about her even to a little 6 year old like himself. Climbing off of his bed, Leo went to follow after his father to go down the stairs and put his shoes and coat on for kindergarten. It was difficult not to be so anxious though when they were actually outside and were on their way there. Of course Sans tried to make it better for him once they were there, by giving each of them his usual goodbye, and wishing them a good day. Leo could only really hope he would have a good day.

With a small smile, Sans watched Leo and Luna head inside the kindergarten, but his smile faded a little when he thought back to that earlier conversation. Don't run away, was what he said, acting as if he still didn't do that. Because he did run away from things still, even though he tried to act like he was tougher than that. It was like whenever someone asked him why he hadn't proposed to Frisk yet, and he shy-ed away from the subject like he didn't want to talk about it. Maybe he just treated it like it was a bigger deal than it actually was, he thought as he was starting to walk back home. Him and Frisk had already dedicated their lives to each other for the last almost 7 years, it probably was like they were already married. 

It wasn't that he didn't want it, it was just... Him being his usual self. Overthinking, as usual. He knew if he proposed, Frisk would very, very likely say yes. He knew that if he proposed, she would be overjoyed, and he could practically imagine the way her face would light up if he considered to get down on one knee. He always knew that he wanted to spend forever with Frisk, he never wanted to be separated from her in a million years. She was the only person he ever wanted to be with, for the rest of his life.

So, maybe he should just stop running away already.