Personally, I would be afraid to take a really young child on the water mice. Once on the Seven Seas Lagoon, DH's water mouse stalled. He was in the direct path of the ferry boat!!!! He did the first thing that came to his head and jumped out of the sprite!!! He got picked up by one of the lifeguards who towed the sprite in, but I can't imagine what he would have done if he had had our DS with him at the time.
Luckily, no one was hurt, but I would NEVER take a young child out on one of the sprites.