We have two trailer bikes. Both are Trek Mtn Trains. Last year there were only two color choices, but I think this year there are more. The Trek Mtn Train last year was the same bike as the Gary Fisher trailer bike for what it's worth.
The Trek has an adjustable stem that is a nice feature.
We've caught our five year old boy riding with his feet on the handlebars and trying to hop off before we are stopped, etc... so you may want to consider buying a rear view mirror.
I think there is a water bottle boss underneath the downtube, which we don't use. I just carry the bottles on my bike and we stop for water occasionally. I like the idea of a camelback that the earlier poster mentioned, but our kids would probably use them as squirt guns.
We bought $10 bike computers for each of the trailer bikes. This not only provides something for the kids to do but is instructional as well. Since there is no front wheel you'll have to mount the sensor on the rear wheel. We just barely had enough cable to run back there.