I am afraid so. I have seen seven year olds scared to death. If the sting rays smell the fish food on your body, they will tend to try to suck it out of what they think is your hand. Although the water is beautiful, I would skip this tour unless you can be assured of their safety. Everyone wears a lifevest, but the tides change and it could be deep for them in spots. Teh water on the sand bar is usually up to an average adults waist. Seven Mile Beach is a beautiful beach to take the kids to.
I think it depends on their personalities, but there's no doubt that they would need life vests and constant supervision. I'm 5 ft., 7 in. and the water was over my head when I first entered the water at Stingray City, though it is less deep on the sandbar.
Often, children are very much influenced by the reaction of the adults around them. I will warn you that MANY adults screamed bloody murder when they entered the water and the stingrays first started swarming. Within five minutes, everyone calmed down and had the time of their lives, but if you think the initial reactions would upset your twins, you should pass.
Besides a beach day at Seven Mile Beach, a good excursion for children might be the Cayman Turtle Farm. Other than that, there's not much to see in Grand Cayman.
Try taking the tour of Grand Cayman. When we did that a few years ago, we went to the turtle farm, Hell, and also went on the glass bottom boats so that we could see the stingrays. Swimming with the rays is a wonderful experience, but not for children that young. Have a great time, I love Grand Cayman.
Thank you all for your advice! I think we're going to pass on the Stingray City Tour with the little ones. The tour of Grand Cayman sounds like fun. We're leaving in 2 days for our cruise. Can't wait. Thanks again, --Marybeth