My toddler threw one of her shoes off somewhere in between Disneyland and downtown Disney. We did not find any shoes to replace them. There was 1 store on main that had some baby stuff, but that was the only place I can think of. World of Disney might have something (downtown Disney)
I don't know if they sell shoes, but I'd try the kid's clothing shop on Main Street. It's between the Clothier and the China Closet (on the right side of the street).
Unfortunately, this was one item we could not find when we were in the parks a couple of months ago. Actually, it seemed like it was impossible to find much baby stuff at all. You would think that would be another market they would want to capitalize on, but there wasn't much to choose from. Specifically we looked for baby clothes and shoes.
There are no shoes less than size 5-6. Little Cinderella had very dirty socks for an afternoon... I guess she's a toddler not a baby, at 13 months has been walking for 2 months. Note to self, pack extra shoes.