I did a quick search and all I could see were adapters with a quill bottom and a cylindrical top so you can attach a modern stem to an old bike with a threaded steerer.

Do there exist adapters that go the other way?

I am trying to assemble a franken(mountain)bike for a kid, and I have a bike with a complete frame/fork/quill stem/handlebar where the frame is too big; but I also have a smaller frame and a threadless fork, but no stem/bar.

Maybe the best solution is just to get a cheap stem, either that grabs the small bar, or also get a shim?

I don't want to really 'invest' in this bike, as the investment could VERY quickly surpass the value of the whole bike...