My wife and I took our 5 yr. old girl and our 3 yr. old boy. They were both intrigued, but they're young enough that the "plot" was realistic. I'm not sure an 8 and 10 yr. old would be as engaged. I, however, was totally impressed at the athleticism of the mermaid.
Depends on their level honestly. I went with my 5yo and a friend's 10yo and 12yo. The 5yo and 12yo loved everything, the 10yo was kind of meh. They get to see dolphins and shoot bubbles at a bad guy on top of simply seeing a mermaid swim around. There is a story/plot to engage in. There's also the Pirates of Hilton Head eith a similar excursion on a pirate ship. We've done them both and all had fun.
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