Guildford is probably easier to get to, but Coral Reef pool in Bracknell has three good slides, lots of play areas including some for smaller children, and a hot-tub bit that the parents hang out in as well.
They also have a Go Ape, and a science-themed play centre, as well as an actual real forest to explore. It is worth a trip.
[no, I don't work for them!
]