It so happens I've been shopping for a tandem and many makers offer their bikes with child-stoker kits (an extra set of pedals that attaches higher up on the rear seat-post) so a kid can reach the pedals. They're adjustable so that as the kid grows you can move them down.
Here's a link site that has a bunch of manufacturers and sellers:
The child stoker kits are great! I have used them with my two girls and have been very pleased. With a sachs chain that uses the quick disconnect link I can convert it back and forth in just a few minutes so my wife gets her stoker time in too!
One Tandem bike that you might be interested in is the Trek T900. The stoker (rear) seat is adjustable down to 43cm and maybe a little lower than that if you swap out the suspension seatpost for a solid post. This is not one that I would recommend for long journeys however.
Depending on what kind of riding that you will be doing, the LoveBike tandem is a great option. The child gets a front seat and can see more than your back. I have had one for over a year. My kids are getting bigger and I am using a Triple quite a bit, but the kids have a great time sitting up front.
Ron Cooper, Time VXSR, rock lobster, rock lobster, serotta, ritchey, kestrel, paramount
I'm in the process of converting my Davidson tandem for my daugther. I got the child bottom bracket and just got the cranks. I got an adjustable stoker stem and a scott drop in handlebar. The drop in handle bar can be rotated around to provide a comfortable reach for a 5 year old. the adjustable stoker stem has a faceplate so I can easily change handlebars to convert it back.
I've been warned not to expect to go too far in the begining.
You can also try custom builders, if your need is such that it makes sense. We went custom because we are on our bikes a fair amount, all four seasons, and have taken the trail a bike as far as it can go. I would not recommend the Adam's version of same, by the way, to anyone that is using them day in, day out -- IMO they are not sturdy enough in the hitch for this use. The Burley Piccolo is surely a better bet in this regard.
We'll have tandem + trail a bike happening when number 2 son hits kindergarten. My 6 year old and I are proving this winter that we can get rid of one car by riding all winter long -- and we are hoping that Vancouver isn't going to have unusual amounts of snow!
We went custom because it allowed us to get the bike we wanted, with exactly what we wanted on it, in a geometry that allowed our rapidly growing 6 year old to do without the kid-back adaptor but without producing unnatural compromises for my wife either (either as stoker or captain).
Our new frame by Dennis Bushnell is supposed to be off to the painters soon, so I guess we'll be checking out our 6 year old against it fairly soon.