Welcome back, Victoria! Your family will really enjoy the
Seaworld Observer Semi-Sub adventure when you are in the port of Grand Cayman. My daughter and I loved our semi-sub adventure when we experienced it!
Once your adventure group brings you back to the dock, you'll have plenty of time to explore Grand Cayman on your own, especially if you book an early departure for the Semi-Sub adventure. You might consider hopping on a bus or taking a cab to Surfside Beach (part of Seven Mile Beach), just down from the port area. It's a free public beach and they offer quite a few amenities. Umbrellas and chairs are available to rent for between four and six dollars, snorkeling gear, jet skis and kayaks are there for rent as well. Add to those things the beautiful beach and the chance to simply play in the sand and surf, and it makes for a nice way to spend a couple of hours. Last time I was there, there were buses and taxis just outside the port area for about $5 per person each way. At the beach, there were plenty of buses and taxis lined up to take guests back to the port area.
Just keep in mind that Grand Cayman is a tendering port, which means that the ship will anchor out in the bay, and guests will take smaller tender boats to and from the dock. So, this adds some extra time to your travels. The line to get on the tender boat back to the ship can be a bit long, so don't wait until the last minute to head back to the port area. I'd leave quite a bit of time to ensure you're back with plenty of time to spare before you need to be back on the ship.
I hope those details help as you plan, Victoria!