People seem to forget that the Main Bully wants to be a psychiatrist like his dad.
They also forget that his “friends” became friends through blackmail. Even MuscleBully saw how he blackmails people to suicide with no care; so him doing that to his parents isn’t out the realm of possibility.
Dude, there is NOTHING a child can do to harm his adult parents. Blackmail? Presented by a child, noone will believe his word over his parents. Evidence? Unless he's psychic, he can't move that evidence into place where it can be found without his parents knowing and (preemptively) whooping his ass or better - sending him to juvie or a psychiatric ward for studying. He does not act like a normal child would - too composed, too nice (the exterior anyway). Children and teenagers are whiny jerks, if your child isn't like that, there's something wrong with him.
This is only available through extreme enabling and fanatic love, and there's enough studies or statistical cases saying that parents are more than willing to kick their own child's behind to "set them straight".
The unrealistic part isn't that he wouldn't care to do it - that's psychopathy in a nutshell, that's real. It's the part of him
being physically capable of pulling it off. As a child you're dependent on your parents' mercy (unless you're an orphan, in which case you're dependent on mercy of the orphanage caretakers/the streets). They say "nay", your only response is "nay", unless you want to be beat or locked up in your room without food. And japanese parents are known for considering slapping their children for speaking out of turn completely normal, I wouldn't be surprised if beating kids is justified in their case with "look what you made me do".
Like in the scene we saw - what exactly is stopping his parents from overpowering him by jumping him, two at a time? What's the best he can do with that skewer - poke out an eye, that's if he's lucky. Great, we get
an assault charge by a child, and not only both parents are there to testify against a child, but he's also of age legal enough to go to juvie. This scenario would've occurred hundreds of times by this point, the fact ONE of his parents is so obsessed with him is already going over the realism plank, if author made both of them insane it'd be...... too insane.