When looking into kids fishing waders most of the major brands have something available. It all depends how much you want to spend. A higher priced wader might last longer but will the child grow out of them before they wear out?
Get something comfortable and easy to move around in. When my son was younger it made all the difference having him in waders. They are able to climb around if they get bored and still stay dry. Your goal is for them to want to put them on and come fishing with you, right?
Here is a list of waders available from the major retailers.
Check back often as I add more waders to the page.
Top of Kids Fishing Waders