One might say, if you're doing something sexual and they're crying and screaming, then maybe it's abusive, but on the other hand, kids cry and scream over all kinds of stuff. They can also pretty easily and quickly stop crying and carrying on sometimes.
Or, one might say, if you cause them physical harm, that's abusive; but, on the other hand, parents encourage kids to do all kinds of stuff that causes them physical harm. If you encourage your kid to walk or ride a bike or play sports, probably at some point they'll fall down and hurt themselves, maybe badly. Typically, they heal from it.
One might say, "If they have ongoing emotional trauma, that means they were abused." But kids can get fucked up emotionally over all kinds of stuff that the law doesn't recognize as abuse. Maybe their parents split up, or maybe they fall in love and get their heart broken. Maybe their peers treat them like shit and they feel lonely and like an outcast. Society mostly turns a blind eye to that stuff, or acknowledges it's less than ideal but still opts to do nothing about it, saying kids just need to learn to deal with it.
There's probably a double-digit percentage of schoolkids who get bullied at some point or another. You won't see it too much in the headlines, unless it was motivated by racism, transphobia, etc. There'll be big headlines if a guy gets caught with a photo of a kid doing something sexual 20 years ago, though. They consider that a bigger deal than kids getting their heads shoved into lockers or pushed into a toilet or whatever, every day, regardless of how much distress that causes them.
They just say, "If you don't stick up for yourself, it's your own fault if the bully does that stuff" although of course if the kid does try to fight back, the bully and his friends tell on him and he gets punished; meanwhile, if he tries to tell on the bully, the bully and his friends deny anything happened, and the teacher has little choice but to believe what the larger group of kids say went down, if he didn't witness it personally.
We might say, "If someone's doing something really gross to a kid, I'm calling to call that abuse" but in some of these scenarios, the kid may not have even understood what was happening or attached the kind of meaning to it that we would, to be able to be distressed by it.
"Abuse" is just an interpretation of what's going on in certain scenarios, and it's arbitrarily only applied to certain categories of behavior that people don't like.