My BF and I had this situation flying in Y from (horrors) MEL-LAX. Behind him was a five-year-old girl who spent about 13 of the 14 hours in flight alternately kicking the seat and pounding full force on the tray table. Multiple appeals to her completely ineffectual and mostly uncaring mother had a slight, VERY slight, positive effect after about the fifth time we turned around.

My idea, which my BF didn't want to do for some reason, was to switch seats with them. (There were two of us and two of them, so it would have been easy.) Then we could have had fun kicking the mother's seat and pounding on the tray table behind her for 13 hours so she would get something of a lesson in the Golden Rule. I think my BF vetoed this because he didn't want to destroy the peace of the person in front of us, too. Overly noble IMO!