Actually, I'm already using BMX cranks on the rear of my Bike Friday tandem, and they are a great solution. In particular, there's a crank made by Sinz that comes in sizes from about 130 to 170mm for square taper and ISIS bottom brackets, they fit 110 mm BCD chainrings, and they can be found for $60 or less online. They are great for tandems on which you don't have a front derailleur (mine uses SRAM DualDrive). In the case of a bike with a front derailleur, I'm not sure whether the BMX cranks would work or not, but it looks like Origin8 makes a 155mm triple that's inexpensive. Unfortunately on many newer tandems, I probably also have to buy a bottom bracket to go with it, but it may be a good option until the kids' legs get a little longer.