Could you regift anything from the bag you just received, or things you've gotten in the past? The anonymous part works for you here, no one has to know it's you regifting.
That might depend on your children's ages, though. Are they young enough where you could take the stuff without them really noticing? If not, are they old enough to help make something or "build" the boo bags themselves from their own toy collections?
Final idea: kids like stupid stuff. If I got "booed" and had to respond using things in house right now, I'd do the following: loose Band-Aids (why do kids love Band-Aids so much?), stickers, candy from the bag I bought to give out, a few pieces of small plastic crap from last Christmas that will never be missed (I'm looking at you, jungle animals), and a plastic baggie of marshmallows labeled "ghost poop".