While both water parks at Disney have areas for little kids, we really liked Typhoon Lagoon. I have teen agers and pre-teens and they thought the "big" slides were quite thrilling.
There is also a family slide, where you go together, and smaller slides, depending on how adventourous your 2.5 yr old is.
The wave pool is very cool, just don't get in too deep. The waves are big, but if you stay at the beach end, its like being at the ocean.
There are sandy areas too. And even though a 2 yr old can't swim in the shark reef, you can all walk through the "sunken ship" and watch the fish, sharks and those swimming with them.
Since the park has a lot of trees, you can usually find some shade when you need it.