Hmm, that's actually something I can't quite answer just yet since everyone will have a Halloween outfit in the game and I want to keep those a surprise... but I've shown a few outfits the characters have worn as kids in short stories and art I've made:
Carlos wore an Iron Man costume made by his sisters while his twin wore a cheap mummy costume he slapped together.
Chris wore a witch/vampire costume (he got excited and merged both together)
Ben dressed up as a mad scientist as a kid.