Children enjoy being afloat; on the move there is always something to watch, and careful planning will enable you to visit an amazing variety of places of interest along the route.
Older children will enjoy helping at locks, but should always be supervised. In most canals the water is not very deep except at locks, where the rise and fall can be 12 feet or more. Even if the children can swim, lifejackets and a watchful adult eye are essential at locks. Don't let the children run around the lockside or climb across gates.
Make sure they understand how locks work and don't leave children struggling with a paddle.