I do not have the answer nor the lead however two places to check are Hodgemen waders and Ducks Unlimited. Give them a call and ask it can not hurt to ask them direct and who knows they might be able to custom make a pair which would require sealing off the leg at your desired length.
When my son was 5 he would hunt timber with us and we used a narrow boat to pull through the timber, once set up I would hang a deer stand on a tree about a foot above the water for him. The narrow boat was great and the stand allowed him to go into the timber where he would no be able to otherwise.
Please let me know if you find out anything on the waders as I know several people who would love to hunt timber again.