If you're staying on-site, you'll want to have your tickets put on your room key, so they'll be hard plastic cards that you can't punch a hole through.
If they're wearing pants or shorts with pockets, just have them put them in there. They shouldn't fall out. If you do have the paper tickets, for peace of mind, copy the front and back of each ticket. They have ID numbers on them, so if a ticket gets lost, a CM can cancel out the old ticket and issue a new one with the right number of days left on it.
You can also buy a thing to wear around your neck to hold your tickets, but teens who don't want to wear fanny packs probably won't want that, either.