Tough cookies, kids. If it's a) too small for all of you or b) too broken to be repaired with a knot or a safety pin, out it goes. And I organized the stuff while I was at it. Biggest drawer (bottom) contains actual costumes - dresses, skirts, tops, pants, like that. Middle drawer (slightly shallower) has accessories - hats, purses, wigs, wings (not many of those left; they break easily), swords, wands, crowns, belts, collars, etc. Top drawer (very shallow) has jewelry and hair stuff - headbands, barrettes, brushes and combs. Then the basket on the counter now holds what they prefer to call "makeup". At their ages this consists of hand lotion, emery boards, sparkly clear nail polish, lip balm, and rub-on tattoos of ladybugs, plus things like mouthwash and toothpaste and dental floss.
When I was done, they were actually pretty relieved that I hadn't just tossed everything, and Abby's organized soul was excited that everything had a place to go. Of course, they have not yet seen what I did throw away, because they're cleaning their room. We shall see what transpires the next time they go in to actually dress up.