I have a couple of young kids who are old enough to start road biking. I also have a spare 650c wheelset and enough components to contemplate putting together a small bike for them. However, I have been unable to locate frames specifically for 650c wheels. I'm not looking at anything terribly expensive since it's just for the kids. But I'm curious if there are complete frames or even lug sets that are available for a bike of this size.
Related thought: From what I can tell there is only about a 6mm difference between the radius of a 650c wheel and a 26" mountain bike wheel. Would it be reasonable to use a mountain bike frame for such a project?
I don't know if it would suit what you want, but Kogswell was doing some 650C frames. Might be nice for you anyway!
Lug wise, I haven't built a 650c, but I don't seen why you couldn't use standard lugs. Where that can get iffy is in making same size frames, but with a different size wheelset. But if you are scaling a frame down, that otherwise could use a set of lugs in the larger size wheel, then I don't think you have a problem. One is scaling down all the parts. On the other hand, trying to make a lugged touring bike with frame lugs designed for 700c with 26" wheels can get interesting. To maintain the same BB height, with smaller wheels, the BB lug angles flatten out.