This pull buoy is the best I've found for young swimmers.  It's still a little too buoyant for my 5 year old, but he can make due with it.

I had one of these for adults and always had problems trying to keep my students from losing it.  It was too boyant for their small bodies.  I'm glad I got the small size...for the students that are at the level to use it, it's an invaluable tool.