To get back to the main issue:
You Kick the Back of this Seat Again & I'll...........
Politely offer to exchange seats with them.
If everyone who was bothered by the kicking followed suit, the offending kids would eventually get to the bulkhead row where they couldn't bother anyone else. I know this wouldn't work in many cases, for example from anywhere behind the exit row, but it might be worth a shot.
Scowling and growling at the kids or their parents (who may well be doing their quiet best) only makes people angry and upset and less likely to cooperate.