My kids are 6 and we currently ride a Trek T900 tandem with a MountainTrain trailer bike (dd is smaller than ds, so she usually gets the MountainTrain). I haven't toured with them yet, but I do at least plan to ride to the local camping sites this summer for weekend "trips" to give them an idea of what my brothers and I did with our dad. DD wants to ride her solo bike around town, but riding our mountainous terrain all the way to the Nat Forest sites is too much for her yet. Of course, the TAB is a single speed, so she gets a free ride when we are going any speed. DS is too busy watching the world to want to navigate for himself, but he is a powerful peddler when he wants us to go faster!
After we get accustomed to local trips, my plan is to drive to a location, set up our camper, then day-trip around the area. I'm not quite ready for an unsupported tour. ;-)
Love your site. Alaska to Argentina sounds like a great trip!
Catherine