For kids bikes, Kenda makes a reasonable road tire. Many wheel chair racers run 540 size wheels & tires.
The link for the Qbikes source should still have stock of these rims. As noted above, I bought a set to build only a couple weeks ago.
So much revolves around the frame you have to work with. It is really nice to have wheels that are the right size.
I built mine on fine Campy Record hubs, too.
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1959 Hilton Wrigley Connoisseur (my favorite!)
1963 Hetchins Mountain King
1971 Gitane Tour de France (original owner)
* 1971 Gitane Super Corsa (crashed)
* rebuilt as upright cruiser
1971 Gitane Super Corsa #2 (sweet replacement)
1980 Ritchey Road Touring (The Grail Bike)
1982 Tom Ritchey Everest
(replacing stolen 1981 TR Everest custom)
1982 Tom Ritchey McKinley (touring pickup truck)
1985 ALAN Record (Glued & Screwed. A gift.)