I was once on a flight with kicking babies in arms. Not one but two. In the end we learned that it was against FAA rules to have two babies in arms in the same row - not enough O2 masks. Further, the FA can only do so much. And unless there are other places to sit your are pretty much screwed.

However, there is a final option - tell the parents that after the next kick you will be informing the pilot that you have been assaulted and request that the plane to met by Airport Police as you will be filing charges against them. If that does not get their attention nothing will.