I hope to follow your thread here to gleem some information. The route I am going is I bought a Burley Samba (circa 1995) MTB tandem off of Craigslist for $275, and plan to add a Burley Piccolo kids add on so my wife and 6 yr old can go on rides.
If my wife doesn't go for the tandem adventure then I can move the Piccolo over to my Cross bike which has frame mounts for rear rack and fenders.
YouTube video of Piccolo: http://www.youtube.com/watch?v=FumaSVX0nAo
Burley's site: http://www.burley.com/home/bur/page_776/piccolo.html
Awesome review from MTBR Forum:
"We used a Burley Picolo to do a 100+ mile bike camping trip in Alaska on a pretty rough dirt road and some short trails. We could load heavy panniers on the steel "Moose" rack and even had a rack on the rear of the Picolo with overloaded panniers.
The Picolo worked great, tracks with confidence and never developed any slop in the linkage to the main bike. I actually preferred it to a tandem because it keeps the kids weight lower as they lean to the side to look at stuff and talk to their brother, etc. Because it is lower, it is easier for the child to mount and dismount too. Unlike on a tandem, the kid does not need to have the same 70-90 rpm cadence you will be maintaining. I kind of let my son save his strength so he was fresh when I needed help on the hills.
At other times I also pulled a kid trailer behind the Picolo, and so long as it was not SEVERELY steep down hill, it would work great, but make sure more weight is on the Picolo then the trailer. If regularly hauling a trailer, a tandem probably is preferable.
In short, I highly recommend the Picolo! Get the geared version. The kid's pedalling effort really can help and it is a great way for them to learn to shift before cutting them loose."
There is also this tow bar: http://www.trail-gator.com/