Footwear is a tough issue for youngsters. You hate to spend $60 on hiking boots, because kids outgrow them so fast. Then, as you mention, you can get into problems with the fit. The market for children's hiking boots is small to begin with, and I'm not aware of any manufacturers who actually make shoes in the width and size you need.
One possible solution: Simply try several makes of shoe. Footwear differs from maker to maker in terms of whether it's true to sizing and how much volume is inside the shoe. So you might find a standard-width shoe that fits your son pretty well. Maybe High-Tec's Chimera Lite ($35), for instance. Or L.L. Bean's All-Terrain Hiker ($38).
Barring that, for casual hiking I don't seen any problem with letting a youngster wear just a comfortable pair of tennis shoes. Even on pretty rough terrain, that may be a better choice than a heavy hiking boot that cuts down their foot "feel" and may increase the chance of tripping over a rock or root. Plus, the fatigue factor will be less. As for support, it's true you don't get the same support as in a pair of boots, but a five-year-old probably isn't carrying much weight anywayin my view, aside from maybe a jacket in a knapsack, I don't think there's any reason to burden a kid with anything.
One rule of thumb: If you're out hiking with light shoes, take a spare pair, as they'll likely get wet.
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