Hi Kristie. We were on the 10/6 Galveston sailing with a 2.5 and 7 y.o. We did the GC turtle farm / dolphin encounter and I wouldn't recommend that one for your group. The turtle farm was brutally hot the day we were there. But even if the weather was milder, I don't think that the farm is more than about a 30 minute experience. It just didn't hold the kids' interest.
From the farm we went over to the dolphin encounter. I sat out with the 2.5 y.o. and it was a struggle for me. There is a very small play area, but the lagoon isn't fenced off in any way and I spent the hour trying to keep the toddler out of the water. With that said, my wife and older son had a great experience with the dolphins.
From what I've heard, the dolphin encounter at Cozymel might be a better choice, as they have more things for the people who aren't encountering dolphins.
For our Cozymel excursion we went to the Nachi Cocom Beach Club. It was an awesome beach experience (if a little pricey) and all ages had a great time. Even the cab ride out there was an adventure. The 7yo had no idea how to roll the window down and we realized he'd never seen a crank window handle
Hope you guys have a great time.