We did the dolphin encounter in Cozumel back in August through
DCL. While not fantastic, the kids really enjoyed it.
The only problem we had was staying on the platform. Depending on the time of day, the current tide, and your height, the water could be waist deep or closer to shoulder deep. In addition, the platform is slippery since it is covered in algae. All there is to hang on to is the chain link fence behind you. With 5 and 4 yr olds you will need to hold them. This can be difficult as you hold the child and hang on to the fence, especially if the water is a little rough. Also remember that this is salt water - not all kids like salt water, especially if some gets splashed into their eyes or mouth.
That said, our boys (5 and 12) did really enjoy it. Mom and Dad were a little too busy holding DS5 and hanging onto DS12 so they would not float off the platform. Neither one could stand and the water was a little rough at times, especially when boats would go by. Both boys also had some "discomfort" with salt water spray/splash but not enough to ruin the experience.
In retrospect, I think we would have enjoyed something like Discovery Cove in Orlando much more. The enclosed area and constant water level is helpful for children. But for a quick interaction, Cozumel was ok.
Try to check on tides and book when low. We did the first one (10:20 depature). Maybe one of the later times would be better.