Don't worry, diggin' around for some kid's used, leaky old waders, when ya' can buy you son a "brand new" pair of nice waders for just $29.00! That's what I did for my son, and he loves them!
Albright Bridgestone Children's waders are the best deal on the web...
http://albrightflyfish.com/items/bri...ldren/list.htm
Bowfin47
PS
Albright even has a sizing chart! (We bought a pair that was one size large, and they worked out great!)