EVERYTHING is on the expensive side on St. Thomas.
We did the 12-27-03 Eastern on the Magic, and took our kids to Coral World, they were 12 and 16 at the time. Wife and I went to Coral world on a previous cruise, pre-kids, so 16+ years ago, and we liked it then, and our kids seemed to like it .
It isn't as nice as say Monterey Bay Aquarium, which we've been to several times, but in the perspective of EVERYTHING is expensive in St. Thomas, I have no regrets about going.
But we got a package deal through the ship that included transportation, and Island Tour, and shopping stop.
I don't think I'd do it on my own, only with transporation included. So, for your age kids, I'd say it's a good trip.
Skyway is fun too, but at $15 per person, expensive, but great view of the harbor and ships from the top. But, if you take the Coral World/Island Tour, you'll get a similar view from the other side of the bay,and save $15 per person.