Of course they are. Is a thief who only steals from thieves, a thief?
Look I don't have kids (yet) and I am not pretending to know the perfect answer here but it is my honest belief that going straight to assault is not the answer no matter what anyone says. Especially so when the assault is on a kid and the assaulter is a fully grown adult.
I was bullied a lot as a kid and honestly I wish my parents/school had better prepared me for that happening. This could be achieved through bullying awareness classes and also self defence classes. Both could teach a child how to mitigate what is happening and stop it getting as far as this situation has. Bullys prey on those they see as weaker than they are, but there are ways to show you aren't weak and stop the behaviour. It is just a shame that kids aren't given the tools to do this more often.
Also you don't even know if belting this kid will even achieve anything. How do you know he won't skulk away after that beat down, and then escalate things on your child as soon as you aren't around? How would you feel then if the next time your kid ends up in hospital or worse? Also you potentially end up with a criminal record which could impact yours and your families lives forever. But feel free to continue thinking it is the best course of action.
I'm a firm believer of what goes around, comes around and I also can't stand bullies.
I loved giving them a taste of their own medicine.
Good example was 3 weeks ago, some little shit kept annoying my son because he couldn't stop my son scoring in the corner. My son started to get upset, I moved my boy to the other side of the field to keep distance. The little shit followed and kept teasing him. I told my son not to tease back or hit him and just smash him in defence as payback.
I actually went to the sideline, ask who's the parent of the kid, and warned him I can't control what my son does to him. 3 or 4 tackles later, my son cracked him in a legal tackle, looked at him on the ground and just stepped over him and joined the line of defence. After the game the dad "tried" to bully me and I just replied "I can drop you quicker than my son dropped your son". Then his own team supporters told him to basically shut the fuck up.
That is the best way to deal with bullies.
As you know I'm not a lefty, not a pc or soft ****. If anyone tries to bully me or my family, I will fight back without giving a fuck.
Don't start nothing and it won't be nothing.
PS as you say you were bullied and I was the bully of the bullies. If I were the one to bully your bullies, you wouldnt be talking like this now. You would be thanking me for taking my course of action...