It really depends on whether or not you are needing costumes for a boy or a girl. For a girl, you can NEVER go wrong with choosing to have your precious little girl in a Disney Princess costume. For a boy you might try a pirate.
There are many classic costumes you can use from Disney World. Any sort of Princess, Snow White, Minnie Mouse for girls. Or for boys you can use the Seven Dwarfs, Mickey Mouse, Pluto or any number of their other characters.
All of the Disney princesses are great costumes. Also, you can't beat a classic mickey or minnie mouse costume. Take a look at the Disney Store online. It will have great costumes. If you can't afford them, you can at least get ideas for ways that you could possibly do it yourself.