Rod,
Crank shortners can be seen here:
http://www.tandemseast.com/parts/cranks.html
If you have any aerospace machinist friends, you might be able to put together some of your own.
Or, maybe find some shorter cranks so their legs don't have to stretch as far when the crank is down.
Blocks on the pedals, or extra-thick pedals (effectively raising pedal surface up toward the stoker is probably a simpler way to go.
Might be time to locate the nearby "Mr. Fixit" bike repair shops and check out their parts bins.
Your kids look to be having a great time. Better start looking for a quad, so you can have 3 stokers plus the trail-a-bike!
- Joel N.