I was going to suggest dead houseplants, but someone already suggested something similar.

Its been a long time since I threw a themed party, but if you have any books, or book shelves in the public areas, I used to like putting paper bookcovers over the books decorating them, and then giving them funny theme related names, usually twists on classic novels, i.e. instead of Vanity Fair, the title would be Vampire Fare, Zen and the Art of Dungeon Maintenance, the Blood of the Baskervilles.

OK, sorry, those aren't very good examples, I'm blanking out on something really clever, but you have time to think up catchy titles, and kids would like decorating the temporary book covers, so its a way they can contribute, too, and be kept occupied...