We did the Stingray City tour through Captain Marvin's and it was the highlight of our day in Grand Cayman. We booked a private charter, but I know several others who booked the regular tour and had a great time.
Even though your kids are not afraid of animals be prepared for them to be a little bit "freaked out" when those stingrays come along. Once they get used to them they will be fine, but the first exposure can be a bit unnerving. My DD11 started out clinging to my back (and I mean *clinging*...I had the nail marks to prove it!) but after a few mintues she was holding, petting and feeding the rays.
IMO, Stingray City is definitely a do-not-miss excursion.