Dear Deadhand,
You never see this recommended. and I don’t regularly wear them. ……..But honestly, just get some cheap neoprene. Not the kind with a boot already attached. This way you can still select decent footwear of your choice. They stretch and keep kids warm. Waders are an especially disposable item for kids. Kids end up wandering around on shore looking for frogs and bugs sometimes and it’s nice if they are warm enough to not want to go home early.
If a kid is too cold have them walk around and do a few jumping jacks. They will be usually sweating in no time. If a kid is too warm in neoprene then they can probably darn near wet wade. Or get a cheap lightweight summer pair of breathable waders if you are hiking miles and only fishing in warm weather.
I have multiple junk Walmart, neoprenes from middle school I still let a few of my (not serious fisherman) friends use. Sure they leak and fit tighter than a painted on wetsuit. But even my non-fishermen friends can last all day in cold water with them. Even with the leaks. Even with the poor fit. They really stretch. Walmart used to have them cheap. I never see this suggested, but if it is a kid, it’s hard to go wrong with cheap bootie foot neoprenes.
It isn’t a good summer solution, unless you are standing in ice water, but the other three seasons it’s a cheap and effective solution. I personally think (just my opinion) it is far too easy to spend a lot on expensive waders. With very little gained for a kid. No matter what you get him, they will likely be beat up and leaking in no time. Especially with a kiddo wearing them. Save your money and get something cheap. Gather feedback from your grandson once he wears them a few times and then adjust from there. Get an expensive pair once he is done growing. Just my 2 cents.
~5footfenwick