I got my kid stoker setup from chucksbikes.com for $85. Catches - he only had 1-3/8" and I have a 1-1/4" OD seat tube. No problem, I cut a shim out of a flexible 1.5" drain pipe (PVC?) and it works perfectly.

Also, the chucksbikes.com setup does not use real kid stoker cranks, just 140mm BMX cranks on backwards. So I reversed my pedal spindles so that my kid could ride with toe clips. I haven't had any loosening problems, though.

Real kid stoker options with cost more money. Good luck!