There are many things other than Stingray City to do in Grand Cayman.
The tenders will dock only a short distance from the main shopping strip on the island and the many shops there can provide hours of fun. You don't even have to buy things to enjoy them. Try on wacky hats and shirts, pick out wild tattoos or henna designs and have them applied (the removable ones, of course), or hit all of the samples of rum cakes! I think you could easily entertain a 5 year old in this area.
There is also a small museum near the shopping district. Some of the exhibits are interactive and would probably hold the attention of a 5 year old as well as inform!
Hope this helps!