How about UT US UF SF come across their kid being bullied only to find out this has been going on for sometime.The reason they didn't say anything is because the kid was trying to be tough like them. (Bouns if the kid starts crying at the end.)
UT!Sans: He’s pissed. Look, do what you want to him, but leave his kid out of the equation. He tells the kid that it’s not weak to tell him something like this. Once the kid calms down, he calls up the parents of the bully and lets them know what their kid is doing. If the parents are apologetic and promising to fix it, he calms down and gives Toriel a heads-up to maybe just keep an eye on them in the classroom. If the parents deny it, or say that kids will be kids…..the passive aggressive wrath of PTA Sans will be heaped upon their heads.
UT!Papyrus: He comforts the kid as best he can, and tells him that they can tell him everything, and that won’t be weakness. He takes them to have a long talk with the bully. If that doesn’t work, the parents. If that doesn’t work, the principal, and up the chain of command until it works.
UF!Sans: Cripes, he was hoping the unwillingness to express emotions would be something he didn’t pass on. He’s unusually gentle with the kid while he’s calming them down, and tells them they need to tell him when shit like this is going down. He actually stays pretty calm when he’s in front of the kid, but then goes to his s/o and just rants. He can’t attack the bully, but there is a very large part of him that wants to. In the end, he just shows up at the parents’ door and tell them he can’t speak for the consequences. Oh, he won’t do anything. But while his s/o may have put the kid under strict orders not to abuse those powers, his kid knows how to fucking defend themselves, and the next time it happens he’s given them permission to break a few minor bones.
UF!Papyrus: Normally he’s all for emotional restraint, but his kid is his little heir. Nobody gets to mess with them, not if he has anything to say about it. He calms the kid down and thanks them for telling him about it. He makes a special point of dropping the kid off at school the next day. He fixes the bully in a gaze, long, deadly, hard…that’s a gaze that’s going to haunt their nightmares. After that, all his kid has to do is threaten to call their dad, and they back off.
US!Sans: He tells the kid he understands a need to preserve their reputation, but in this case, it’s absolutely okay. He invites the bully over with their parents to the house (while his kid is with Asgore or Alphys) and has a long talk with them about it.
US!Papyrus: He just sort of holds the kid for a while, letting them know he’s gonna take care of this. He has a talk with both Asgore and the principal about it, asking them to keep an eye on them, as well as get in contact with the parents. If he gets any wind that the parents are ignoring this or brushing it off, they are gonna be on the receiving end of a long and highly sarcastic email.
SF!Sans: He compliments the kid on wanting to be tough, but lets them know that in this instance one needs allies. He teaches the kid a few moves to defend themselves, and tells them that if it happens again, throw an elbow. If they do it once, they likely won’t have to do it again. He also lets the principal know.
SF!Papyrus: It kind of breaks his heart, honestly, and he tells the kid that it’s not a weakness to tell him about stuff like this. He talks to the teacher and the parent, and for a while he makes a point of picking up the kid and dropping them off. He makes long, menacing eye contact with the bully every single time, and hiwle he’s not quite as terrifying as Boss, you try dealing with 6′11″ of pissed off edge staring right at you.