We all have dealt with something like this probably or will in the future. The solution that I try and negotiate for the child kicking my seat is to talk to the parent and tell them,

"I know it is tough trying to keep your young child in that seat without moving for this whole flight. I certainly don't want to have my seat kicked all flight and I know you don't want to have to keep reminding your child to stop doing it. Thus let's ask the FA to move you both to the first row, that way there is no seat in front of him to kick and you can rest easier knowing that is one less thing to have to worry about?"

Proposed it 2x's and only once did it work for the people in row 1 didn't want to move, and I didn't blame them.

I think it was a good solution and when and if I have children some day I am going to remember to book the first row if I feel my child might get restless.