I'll 2nd Bill's suggestion. I got my son (11) squatting well below parallel in pretty short time by getting him to warmup with sets of (among other things) OHS w/ a long stick.
For whatever reason, it seems to help you focus on form (back straight, body straight up/down, heels firmly planted, head looking straight/slightly up).
Prior to starting, Christopher couldn't even get to parallel in good form.
Lots of good stuff in the Dec '02 CF Journal about squat form as well.