We make our Scouts and Cub Scouts bring their packed gear to the troop or pack meeting the week before the trip. Everything gets pulled out, checked over and repacked, Then we throw the packs into the troop trailer. They then have one more week, and one more meeting, to be sure they have everything. Then they have to root through the trailer and repack everything wich is a TOTAL pain. Nobody's forgotten anything yet, at least on the way TO camp. Coming home with everything is a different story.
The challenge we have is with the younger kids. They have to camp with a parent or guardian, so we leave it up the adults to pack on their own, with a checklist from us. Somebody always forgets something or second guesses our list. "Why on earth would we need 6 pairs of socks two nights out?" It usually only takes one trip for the inexperienced parents to make sure eveything on the list is checked off.
Nevertheless, we leaders always bring extra for just in case. It'a vitally important that the kids have fun and it's not their fault if mom and dad don't pack properly.