Thanks for the feedback! We'd like to plan on camping partly because we just like to camp, but also because it gives us a bit more flexibility for distances that we ride--we can pull in early if we're getting tired or the weather is bad, or keep going if we're feeling good. I'll always have my credit card nearby, though, for when we're in dire need of a shower and good mattress. We already own a lot of the camping equipment, although DD will need a new sleeping bag (she's been using one someone left behind in our camp years and years ago, when she was three or four years old!) and we'll need a backpacking tent since the family tent is too huge and heavy. I can borrow that from a friend, though. The fleece sleeping bag is a good idea--probably a lot cheaper than the expensive lightweight bags, too!
The panniers I own are too small for two people, but the good Arkel panniers might be able to hold our equipment. I don't know if I can put a rack on DD's bike--she has a pretty decent Novara mountain bike that fits her well now and should be good on the trail. I'll look into getting a rear rack on it so she could at least carry her own sleeping bag and clothes.
I wrote to a guy who rode the GAP and C&O last year with his kids about training, and he sent a good reply that I'm going to repost here in case someone has this same question in the future:
You should definitely ride the canal with your daughter! Neither of you will ever forget it. My kids were 12, 10, and 6 at the time of the ride so your 11 year old is definitely old enough.
For training, we just spent the summer going on bike rides as a family. Denver has a massive network of rec trails which we took advantage of. We started out small (5 to 10 mile rides) and worked up to large (50 mile all day rides).
To make the rides fun for the kids we did a couple of things specifically. First, we did not emphasize speed at all. We rode at whatever pace was comfortable for everyone, meaning we rode as fast as the slowest person wanted to.
Secondly, we gave each person on the ride 1 or 2 "Pull-Over Cards" (depending on the length of the ride). Each person could "play" their Pull-Over Card whenever they wanted to. That way we could include a lot of kid-initiated breaks for things like snacking, going to the bathroom, resting, playing at playgrounds, floating stick boats in streams, etc. while at the same time the kids had to make some decisions about the best use of their limited Pull-Overs.
I think these two things were the keys to our training success. Instead of training rides they became more like family adventure rides. Our average speed on the trail during these rides (including pull-overs) was around 5 miles per hour as a result.
I need to stop reading journals on Crazy Guy on a Bike. I'm getting no work done.
Sarah