Thanks everyone! We ended up buying Teva's at a local dealer. The boys wore them for a few hours yesterday and so far so good. I'm really happy with how they fit their narrow heels.
Oh and they are waterproof, woohoo!
Here's the style we bought:
http://www.teva.com/ProductDetails.aspx?productID=6281&model=Psyclone&mnu=kids
1 pair in Ultramarine for my 6 yr old, and 2 pairs in Moon Mist for my 4 and 2 yr olds.

Here's the style we bought:
http://www.teva.com/ProductDetails.aspx?productID=6281&model=Psyclone&mnu=kids
1 pair in Ultramarine for my 6 yr old, and 2 pairs in Moon Mist for my 4 and 2 yr olds.