Welcome back, Claudia! Disney Cruise Line offers quite a few different dolphin Port Adventures, and there certainly are some great ones in Cozumel.
While I haven't participated in one of the dolphin swims or interactions at that particular port, I can say that I've heard amazing reviews from other Disney guests who have loved their experiences there. The one I hear the most about is the
Dolphin Discovery Cozumel adventure, which is at Chankanaab National Park. Children ages three and up are able to participate in this interaction with the dolphins, however an adult must accompany them on the same port adventure, so you can take both your children on this one. This is the adventure that you've read about with a submerged three foot deep platform that guests stand on to interact with the dolphins. Bear in mind that all guests will have lifejackets on, as the tide does vary and the water level can change.
If you'd prefer your kiddos to meet with the dolphins in a shallower setting, check out the
Dolphin Kids Adventure at Dolphinaris. This adventure is just for kids ages three to nine. Adults won't fully participate, so you'd only need to book yourself on the
Dolphin Observer adventure at Dolphinaris (which costs significantly less) in order to accompany your kids. Here, the kids also must wear a lifejacket, but they'll be on a submerged platform in water that averages 1.5 feet, though again this can vary with the tide.
Have fun as you continue to plan!