Fishalot I can relate. I have two boys also, they are 6 and 4, and both love to hunt and fish with me. My six year old is the size of the average 10 year old, and is growing like a weed. I have spent many a trip to various stores looking for quality kids gear. The dilemma is what to buy and how much to spend for something they are not going to be able to wear next year. Being the smart and ingenious man I am (don't ask my wife)I devised a 3 year plan for buying gear for my oldest boy that my youngest will be able to wear the following year.
For boots I would recommend one or more of three options. Cheap $20 rubber boots from Joe's. Cheap $30 hip boots from Fisherman's Marine, they go down to size 3. They are a little tall for kids, but will keep the water out. Another option is the kids neoprene waders from Herters, they are $79. They are hands down better and less expensive than anything else I have seen in either Cabelas, Mack's Prarie Wings or anywhere else.
For rain gear, Herter's has a kids PVC rain suit (jacket and pants) that is 14.99 at set. Buy two because little boys and blackberry bushes attract one another. Cabela's offers a waterproof/insulated water fowl jacket made of Cordura. It has their branded version of GoreTex (Dry Plus) in it and comes in Adv. Wetlands. They also have the same coat in "silent suede" fabric in either Mossy Oak Break Up or Adv. Timber. I bought this one for my oldest boy for Christmas. It will work great for duck/goose and deer and turkey. It runs $79. They also have matching pants.
I can appreciate you not wanting to spend any money on new stuff. By the time I outfit my kids, I have no $$$ left for my toys.

I would recommend you check out the hip boots at Fishermans.
My two cents on the whole kids hunting gear scene is "why didn't I think of this!?!" Can you imagine an industry with a guaranteed 100% return customer base for 10-15 years! And this stuff is not cheap!
Anyone else out there, call me in two years and I will have enough gear to outfit any youngster between the ages of 5 and 10. :shocked: