By honestly, I think there's an actual study that shows kids don't actually go out and kill a bunch of people after playing violent video games, and watching violent movies. That should be common sense.
In the case of fighting though, you might have some kids trying out moves, and techniques on other kids, depending on if they have adhd, or just really want yo practice moves on friends and other kids. Sort of like watching WWE. Thats why I say its case by case. But if your kid is a normal, well adjusted kid, let them watch it.
I grew up watching a tom of boxing, and never punched out kids on the playground. At most, I was just shadow boxing at the time. In my daughters case, I could totally picture her trying out some moves on me lol.