OshKosh makes a good and very affordable line of sport sandals for little kids. If you have a Marshall's nearby, they often have them for less than $10.
Bass also makes children's sizes, but not toddler sizes. and Merrell has a line of serious amphibious sandals for older kids. Land's End's new water sandals are available in kids' sizes, too, but I hear that they are so popular right now that backorders are becoming an issue.