We will be on a cruise in Grand Cayman on 1/1/09 on from 7 am - 4 pm. Best places to see and do during this short time. Grndprts want to take grnddaughters to stingray city/snorkel/turtle farm, but parents would like to explore island. Granddaughters are 7 and 2. I would like to include turtle farm.

there are loads of tour companies on gc. this one for example offers one tour that does stingray, snorkeling & the turtle farm: http://caribbeanshoretours.com/Grand...ShoreTours.htm
would grandparents feel comfortable going on the tour with grandkids while parents rent car or get a taxi to tour them around the island?

I would do Stingray Sandbar for sure. It's a great experience, and without a doubt a highlight for Grand Cayman. You could do a half day charter, and then still have time to do some other thinsg as well. GC is pretty flat and scrubby, so not a whole lot of exploring to do. Seven Mile Beach is a beautiful beach, which has both busier areas & quiet areas, depending on your interests.

Agree with the above. You might also want to check out the Queen's Botannical Gardens.

I really believe, based on 3 visits to GC, that for children and the young at heart Sting Say City is worth the trip. But on a tour it may take up too much time vs. a rental car (expensive) and drive the 30 minutes out there for I believe you will only want 30 minutes there. The turtle farm is o.k. but really fascinating for children but it is opposite direction of Sting Ray City. If bored it may be worth going with your rental car. The 7 mile beach is lovely but other than that the island is quite barren so sightseeing is not why you go to GC. You go for pretty good snorkekling, nice beach, nice hotels (Ritz, Hyatt) and a few good restorants.