We have one. Some of the younger children in the neighborhood play on it -- under the age of 5. They have a hard time coordinating the run and belly flop concept of the slip and slide. They tend to run, stop short and throw themselves on their stomachs so they don't slip or slide...they just lay there. But since the water is sprinkling up from the sides to wet the plastic they like it...gives them a gentle sprinkle.
Also, we've found, the lighter you are, the faster you slide. DD (7) is very thin and she flies right off the end...up and over the blow-up lip at the end which is about 4 inches high when inflated. But her friends who is bigger never has that problem.
Let them use it...but keep a really close eye on them.