Welcome aboard, Andrea!
I highly recommend the
Cozumel Ruins and Beach tour for your group. It has a little something for everyone. My five year old son and I did this adventure in January with Disney. Our tour guide was amazing. Even just riding in our transportation to get to the ruins and then the beach was an adventure in itself because our guide had so many interesting things to tell us. The ruins were incredibly interesting, more so for adults, of course, but my son loved it too. Since you'll be there just as summer ends, I would certainly bring a lot of water with you to stay hydrated, as it was hot! Your tour then circles the rest of the island and ends at Playa Mia Beach for a relaxing time in the sun and surf. The kids will love to play in the water while you grab a bite and a fruity drink on the beach.
As for dolphin experiences, check out the Grand Cayman
Dolphin Encounter and Turtle Farm or opt for the Dolphin Extreme Swim if you can, as you'll get more interaction time with the dolphins. Besides an amazing swim and meet with some beautiful dolphins, you'll also get to visit the Turtle Farm across the street which was my favorite adventure ever. Guests even get the chance to climb into tanks to interact with the baby sea turtles.
I hope those ideas give you a starting point for choosing your adventures, Andrea! Let me know how else I can help!