It is cold water, to simulate the ocean the fish live in. If your child can swim in a competitive swimming pool and is comfortable with that temperature, it is a bit cooler than that, but not much. We enjoyed it and are looking forward to a 'visit-long' pass next holiday so we can enjoy it a few times. I swim in a competitive swimming pool and a cold lake often, and still find those cold, so Rainbow Reef was really cold for me, too. I was shivering when we came out of the Rainbow Reef after 25 minutes, but I'd do it again and again regardless. It's a clear and beautiful place to admire the fish.
You can buy a single snorkel session and see if they enjoy it. If they do, you can go back within the hour and pay the difference to upgrade it to a 'visit-long' pass so they can go over and over again. It's close to the prices of two single sessions and they add a punch hole to your card so you can go over and over again. It's a nice place to cool off on a hot day, and even just 10 minutes a day can be fun.