I'm with you here! (I must add, I'm surprised to see someone with the same idea!) Except I'm making it for an adult male. Should be interesting heheh. Altering white footie pajamas is probably the best bet. I'm thinking a fleecey kind of fabric(?) for the hood and the claws and ears. Don't forget the tail! I can't remember if he wears the crown for the entire book, but I'm definitely including it in the costume too! (It might just be easier/better to skimp on that detail for a 5 mo. old though.)
Oh and btw, try searching www.flickr.com
for "max where the wild things are costume". Most of those are children. Perhaps that will help.