You can get one-piece hooded rainsuits with leg snaps, but they are usually pricey unless you are going to get a lot of everyday use out of them. I have one that I got at OUAC for $10, it retailed for $60. DS wore it all the time, but we lived in a rainy climate and I let him play outdoors in rain.
Best option for WDW is usually to just cut down a regular child-size poncho. If you want the sides to fasten, I recommend using glue-on velcro dots. Obviously, this is something you would want to do before the trip, and take it with you; you don't want to have to beg for scissors to cut down a poncho while you are there.