In my first post about my Bike Friday tandem from last year, I showed a picture of the Ride2 crank shorteners I first used for making the rear crankarms a more suitable length for riding with small kids on the back. Crank shorteners get the job done, but they are a bit expensive ($110 on Amazon right now), and they have the effect of spacing the pedals out further away from the bottom bracket (the so-called Q factor). I would assume this is more noticeable to riders with short legs, so I think it's a less than ideal solution. Since the Bike Friday was already set up with single side drive due to the DualDrive shifting, the rear cranks can be replaced with any standard crankset.
When we acquired another tandem last year, I put the crank shorteners on that bike and tried using a short crank on this one. The cranks pictured here are made by Sinz, who make parts for high-end BMX bikes. These cranks are available for both square taper and ISIS bottom brackets, in lengths from about 130 mm to 180 mm in 5 mm increments. They can be found online for about $60, and occasionally less. The ones in the picture are 145 mm ISIS cranks. (Note that the bike comes with square taper bottom brackets, but I found an unusually good price on an ISIS crank and happened to have an extra ISIS bottom bracket.) The cranks have a 110 mm bolt circle diameter, which matched the old cranks, so I just moved the original chainrings over to the new cranks.
I think this is a superior solution for tandem riding with kids. It's not adjustable in length like the crank shortener, which has three different lengths. However, it is possible to buy two Sinz cranksets in different lengths for not too much more than the cost of one pair of crank shorteners. If I did it again, I would do that, getting them in lengths of 145 and 155. Using this approach requires single side drive, but in this era of compact road cranksets and 11-speed gearing, one could theoretically convert even a standard double side drive tandem to use cranks like this without losing much in terms of gearing range.