Little kids have a hard time sitting still. I try to keep my four year old from kicking the seat in front of him, but it's not possible to stop it completely.
If you're talking about big kids, who can sit still, or parents who don't do anything to try to keep their toddler still, that's one thing. But, if you want to ensure the back of your seat is not touched at all, your only choice is the last row of a cabin, or UA E+.
Also, just because you don't overhear the parent behind you saying something to the child doesn't mean the parent isn't trying. If I see my son start to swing his legs, I'll put my hand on his legs and that stops him for a little while. If I don't see him start to swing his legs, it's too late to prevent a little bump or two (little legs in a small space move fast). If you were sitting in the next row up, you wouldn't hear anything, but that doesn't mean I'm not trying.
Analise, when you lie and fake sneeze in this situation, you are being childish. Kids cannot sit perfectly still like adults, but adults can be civilized and not act like children.