Been There...
Hi. Also been a while since I posted but I have certainly been and done this one! My DD wanted to come on a cycle tour with us a few years ago when she was just 10. She had grown out of her tagalong and after advice from various people, a tandem seemed to be the way to go.
I went around to every bike shop I could find that had one and tried them all. They do vary a fair bit in geometry and this can work for you or against you. I am only 5'3" and she was probably about 4' when we were looking (although she's catching me quickly now she is approaching 13).
You get the ones that fold (like your Bike Fridays) which can accommodate large variations in sizes of the two riders, and also will be good for soaking up the growing stages. You get the MTB-style ones which have a little fatter tyres, cushier seats, riser bars (usually), broad range of gearing (MTB stuff) and a more sloping frame which can help deal with different-sized riders reasonably well. Then you get the more roadie-style ones that tend towards more conventional geometry, skinnier tyres and either flatbar or dropbar arrangement and roadie triple gearing (in the ones I looked at). These can sometimes be a real compromise if there is a large discrepancy between the sizes of the riders.
We were going to travel long distances on the road and eventually found and bought the flatbar roadie style of tandem. It felt pretty good, not too heavy and it was a small enough frame that I fitted onto it (with a little compromise here and there) and with a little tinkering so did DD. The only real changes we made were to put a nicer seat on for her (skinny hard little roadie seat did nothing for her) and to drill and tap a second set of holes in the crank arms (the Truvativ cranks were meaty and flat enough to accommodate this) to shorten her crank length until she grew a little bigger. We may also have narrowed the bars slightly for her too. There are pedal kits you can get that sort of put a little block on top of the pedal to make the reach a bit shorter for them but it doesn't do anything for the KOPS position so I preferred the drilling option.
One thing to think about is pedals. It came with toe clips and I replaced them with flats because I thought it would be easier for her to manage. After the first time I stopped pedalling, causing her feet to fly off the pedals and then the pedals collected her in the shins as I re-started pedalling, we reverted to the toe clips. We used these for our first tour quite successfully, but it was quite the dither around getting her in and out.
I eventually found a very cheap pair of shoes (at JensonUSA I think) and put an old pair of clipless pedals on for her. I had the type that had a platform cage thing around the edge and the MTB clipless in the middle left over from my old hybrid, so I used those. She took to them like a duck to water and by the time we came back from our second tour was asking if I could put them onto her own bike. We had another pair in the pedal bucket, so I put them on her bike as well, and she has never looked back. I think it is well worth it if you can manage this, as it really streamlines the getting on and off.
On the transport side, we had a small SUV (Subaru Forrester) when we used to transport it places a lot (now we just ride it there) and with both wheels removed it is about the same length as a conventional bike and so we just put it on the normal hitch-mounted bike rack across the back of the car. Friends with a similar rig, mount it on a normal roof rack (but I am too short to do this without a chair to stand on). I suspect that this is another area where the Bike Fridays come into their own, as a folder must be a bit more manageable to transport.
Not sure whether any of this has helped, but if you have any specific questions, feel free to ask.
PS: I found the tandem (and the time spent on it) a wonderful investment in the parent-child relationship. The conversations were amazing!
Also, I found it a wonderful way to train her in efficient riding and in good traffic behaviour.
2008 Shogun Ninja/BBB Women's Race
2010 Scott Speedster S20FB/BBB Women's Race
2011 Avanti Vitale 3 (converted to flat bar with triple)/Zero Zia Pro
2008 Kona Lisa HT/WTB Speed She
2009 Specialized Era Marathon/Ariel SL 143
2008 Holstar Roadster (tandem)/WTB Speed She