There are some lower ones, similar to the tadpole recumbents, which may be good for a kid.
Actually, quite a few on Craigslist, but most are pretty expensive.
https://www.google.com/search?client...craigslist.org
https://racine.craigslist.org/bik/d/...996750312.html
Nonetheless, I wouldn't be surprised if they would be subsidized by insurance, or similar.
I'll check on that Invacare Handcycle Tuesday AM, and let you know if it is still there. Shipping would likely be a pain, but it could probably be torn apart to pack.
It would at least give something to start practising with, and could donate parts if you wanted something different.
Recumbent trikes pop up quite a bit, but tend to be exensive, but might be a good base to modify.
Our local bike co-op makes what they call their tri-hauler. It is a leg steered recumbent. Several photos of it here, including some construction photos. Obviously a different crank position, but something similar might be easy enough to modify.
Cargo Bikes & Human Powered Machines on Student Show
The handcycles I've seen typically have a fairly normal crankset, and special hand pedals attached to crank arms, both together.
I did see one that was supposed to be seat steered. Interesting concept. I think it was rolling the seat to move the steering.