I went with an 18 month old. The only thing that they can do inside is the kiddie area (squirt people, shallow water, short slides into the water) and the very beginning of the wave pool where it is shallow. Everything else is for about 5 year olds and up, and there are different height requirements for the various big slides. I'm not sure about the outside stuff because we stayed inside.
It is expensive, but you only need one night! Ask when you can get in (I think it is 11 am - even if your room is not ready yet, you can go to the water park part.) And you get to stay all day the next day (just check out and put your stuff in the car). They also have an arcade and story time at night. I did not think that the restaurants were good (and too expensive), so order your own pizza or go out of the hotel to eat dinner. Rooms come with a microwave and fridge, so you can bring plenty of snacks.