I don't own a tandem but I've been researching and posting here about kidbacks for the last few weeks. That will raise the whole back comparment up.

PRECISION TANDEMS - Child Stoker Kit Installation Guide, child stoker, kid-back, kidback, kid kit, tandem, riding tandem with children,

Tandem-Specific Components
As someone pointed out here, kids grow up so these things are often for sale. A couple were offered to me when I posted about it, go down the page and find my threads.
http://www.bikeforums.net/tandem-cyc...nstorming.html
http://www.bikeforums.net/tandem-cyc...ack-ideas.html

There's an alternate way to rig the chain where, rather than adding another timing chain and cranks to the kidback, you move the stoker's crank and FD to the front BB, put the captain's crank in the kidback, and leave the rear BB empty. It's a lot of rigging and not as quick to convert, but all you'd need for hardware is some more chain and quick links, and the kidback bracket. This photo from Sheldon Brown's site shows the basic idea. Because the intermediate crank is gone, you can move the kidback lower than it normally mounted, so the height isn't too absurd for small adults.