Lo's idea is awesome, as are all the others!
Here's mine: Depending on how many children you invite, let them make ragdolls of themselves! When I make ragdolls, I almost always use scrap fabrics, or old clothes, but if you need to you can get some muslin or something, as the base fabric. To save on money and time, they could glue fabric on to the dolls for clothes, or paint it on. I wouldn't try this if you're inviting 10 or so kids, but if it's just a handful, have a mom-friend help you make the plain doll-bases. Of course, provide lots of buttons so they can sew(or glue) on the eyes! You could get a bunch of those cheap little plastic tapestry needles, provide one for each child(as a favor). For functional decorations, fill mason jars(or s'ketti sauce jars, whatever) with buttons(in one), yarn(in another). Or put them in pretty bowls.
Big weird fake flowers(like Birds of Paradise or something) would make cool decorations, too.
Gummy bugs would rock! OOH, and those gummy rats!!!!(Dollar Tree)
You are so lucky that her birthday is after Halloween, you can ransack the clearance sales!!!