just tossing in a few for the brainstorm..
Make sure it is "kid friendly"..
Ie: "i have to go to the bathroom NOW!!", "How am i supposed to eat?", "i'm HOT!", ::drops ketchup globs on costume::... "Uh oh"..., "this itches!"
I'm thinking it would be really cute with a pretend velvet with a cotton liner.. but that can get expensive unless you know of a discount fabric place with excellent selection. I don't think they make felt that large. but it would give it that slightly fuzzy look to it. But felt can tear if you pull on it. Kids are pretty rough on their clothes... cordoroy would be more durable.. I would hit up the thrift store and try to find a big blanket or giant sweater and turn it inside out. Think.. airplane blanket....
You could also go with felt for the tummy, make a few small armpit breather holes... velcro along the bottomside for pottytime... use a baseball cap for the head/hat...
Anime Matsuri: Amaimon(Blue Exorcist), Fushimi Saruhiko(K) & Levi(SnK)
Wondercon: Ikki (Amnesia)
Anime Expo: ?