It's foolish to be subtle with a child. I take a direct approach. I recently had to confront the child behind me at a theater.

I simply turned around and asked if she was planning to kick my seat through the entire movie and if that was her plan, would she please sit elsewhere. Her parents were mortified and the whole family moved.

Glaring is not sufficiently clear. For all the parents know, you're just a pervert of some sort.