We had a great time at the Great Wolf Lodge in Charlotte, NC. It's an indoor water park with a wolf-themed hotel. We stayed for two nights, three days - which was enough time for my kids to get exhausted and fall in love with the Great Wolf Lodge. They ask to go back every few days.
Here are the girls in the small children's play area, which had two lifeguards. The GWL is very safe, with tons of lifeguards: