What are you all talking about? At the
Disneyland hotel, the monorail water slides DON'T end in a pool of water. Why would kids need to wear a life jacket? In case they get submerged in the 6" of water at the end of the slide?
The only DLR hotel that has strict slide rules is the GCH. Kids there cannot wear life jackets and have to be capable of independent swimming to go down the larger tree slide. The smaller slide to the side is open to kids of any age, and parents can sort of wait near the bottom to catch their kids at that one.
The slide at Paradise Pier hotel also doesn't end in water. Anyone can go down that one.