The solution to not using the seat back as a hand-rail is to simply lean on the arm-rest or seat-back of your own row. It's a simple solution, but most people are too inconsiderate to even realize.

The emotion of a parent being offended when a stranger touches their child such as was described earlier (lightly hit or something) can be argued to be on par with the emotion of a traveller being irritated by a child (or adult) being unsociable. I hold the parent responsible, and I would make sure the behaviour was corrected at once. Everyone should be a little more respectful for other people's sensitivities.