One suggestion to end the kid problem: you could always identify yourself as a FAM to the parent, and suggest that unless appropriate action is taken to stop the kicking the parent will be met by LEOs at the flight's destination.

Short of that, you could always mention to the FA that the person sitting behind you is acting suspiciously. Perhaps even suggest that they didn't turn their iPhone off during taxi and take-off (as that would likely be enough to send some of the Naziish FAs into a tizzy).

There are a number of retaliatory things that could be done to passengers who act rudely during the flight and don't correct their actions. Just let your imaginations run wild.