The kiddo is almost 7 (in about 6 weeks). He enjoys our forays on our "Superbike" but does tend to get bored easily (not usually in a ride when there are other riders though). We mostly stick to a local MUP and hit the road on organized rides. The furthest we've ever gone is ~27 mi. at the "Duncan Dehydrator" in Oklahoma. 25 or so is about our (mostly my) limit until we can get ourselves a proper tandem. I'm gonna try and build one up over the winter. We may try the 50 mile route next year.
The Trail-A-Bike is an Adams Starter model. It is a single speed. He can keep up with my cadence until we hit ~30 mph (we do pass numerous singles on downhills and we just coast but we do pay for it on the climbs). It is attached to my '87 Stumpjumper Comp with some street tires. The whole setup is kinda heavy so that's one of the reasons I'm leaning toward a real tandem next year.
He loves the Camelbak (had a Skeeter, now has a new Mini Hawg). I catch him just spitting his water on the ground or sometimes letting it drip from the hose...he has never run out of water though. And I have stressed the helmet issue from the get go so it's second nature for him. Next ride is in October...25 miler.
BTW...Burley (you have a Solo) makes a real nice trailer bike (so I've heard).

Scott