I would gage how your kids will be in the water with fish. My kids are 6 and 4 now and the oldest is TOTALLY freaked out about being in the water with live fish. Also, the water is on the cool side of comfortable and it is salty in many places. Both of these would bother my kids. (who wouldn't swim in the salty pool on a cruise ship). So I would first consider how adventurous your kids are and then go from there. For some, it would be a great escape from the hustle of WDW. For some it is a lot of $$ for uncomfortable, unhappy kids.
My kids have seen the adds and asked to go but we are going to wait at least one more year.