Originally Posted by
Chris_W
One cheap(ish) option is to go with a single-side drive setup, using one of the chainrings on the right side as a timing ring. Then you can put some kid's cranks on the rear (just get a model that has bolt-on rings, not riveted-on so that you can play with the sizing), and get the right length bottom bracket axle to make the chainline similar on the two cranks.
BMX component maker Sinz makes a crank for both square taper and ISIS bottom brackets in increments of 5 mm from about 130 mm to 180 mm. I use them on the rear of my single side drive tandem that is set up for my kids to ride on the back. They used to be available on eBay in all sizes from a couple of dealers, but the selection is smaller now--I'm wondering if they are no longer made. There is also a 155 mm triple crank made by Origin8 that sells for $45.