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.
Those bosses are for a splash gaurd. I run a rental shop and have a few Trek Mountian Trains. IMO, the Adams Trail-A-Bike is easier to maintain for fleet rentals.