Did this with DD and DS aged 9 and 7 when the park reopened after it's refurbishment.
Both loved it, especially DD who's really into wildlife. Biggest problem was getting the mask to fit them, as they seem to supply a standard size for everyone - adults and children alike.
They do provide wetsuit jackets to keep you warm(er) in the water.
My biggest problem was I wear glasses, so in my dive mask I have prescription lenses. As I never brought my mask, I tried using theirs, but couldn't see too much. In the end I used my goggles (with prescription lenses) and their snorkel. Not ideal, but it worked for the short time you're in the water.
Helps if you can use a snorkel already.
Look out for the little hammerhead shark.