Not sure I can help much... I bought a "Tsunami" child stoker kit from the now-defunct chucksbikes.com. The chucksbikes kit is an aluminum block threaded for a BB that clamps onto the seat tube. A standard square-taper cartridge bottom bracket is installed in the block. Cranks are standard 140mm BMX cranks, mounted backwards. This results in the pedals being threaded "wrong". So I swapped out the spindles (L to R) in my pedals and used loctite on the threads. NO problems.

I don't usually carry grown-ups, but if I do, I'll take off the kid stoker (four allen bolts)... I also put a quick-link in the chain to speed up the change.

My cranks, as I mentioned, are 140mm BMX cranks. They are too long for kids under 4 years of age, in my experience. Probably for a 4' kid, 140mm would be fine. You can look at the cranks on typical kids' bikes for comparison.

Setting up the handlebars can be trickier than buying the kid stoker setup. I use Wald 8069 hi-rise chopper bars in a standard stem; they seem to work fine.

I do not know where to get cheap kid stoker sets. You can ask on the Tandem@Hobbes mailing list.