I've done pulled pork a LOT in the last year for big groups. My average butt is 6-7 lbs (before cooking), and I usually figure one butt will feed 10 people (mostly guys). If you have a low count of guys to gals/kids, then it'll go even farther. Guys ALWAYS get seconds, moms and kids usually don't.
IMHO, 4 butts will be plenty for 30 adults and 10 kids, with room for leftovers. I've done the kinda cook where you make something for the kids (hotdogs) and the real Q for adults, but you can't keep kids (the ones with good taste anyways) from getting the Q! /infopop/emoticons/icon_smile.gif