Hi L.! Your request didn't indicate whether you had all boys, all girls, or some of each so I came up with a few ideas and you can sort through them based on the kids' gender.
1. Have them dress up as Star Wars characters (very popular right now)
2. Have them dress up as Winnie the Pooh, Tigger, and Piglet.
3. Have them all dress up as fairies (if girls of course!)
4. Dress them up as bees and a hive.
5. Dress them all up as clowns.
6. Dress them up as Mickey, Minnie, and Donald Duck.
9. two policemen and a prisoner
10. angels/little devils
11. Ninja Turtles
12. Power Rangers
13. Characters from Harry Potter
15. Sesame Street Characters
16. Ketchup, Mustard, and Hotdog