Our kids were small, O.K. swimmers, but the cruise was their first real snorkel adventure. They saw it all at CC. Get them comfortable with the mask and snorkel before you even leave town, it may be cheaper to buy your own stuff instead of renting, it will fit so much better. One thing we use is slap-straps. These are a soft velour band that holds the mask on, much more comfortable and easier to adjust. If you don't have a pool, they can even practice in the bath. I toss coins, shells, beads into the tub, then they try to find them. The hardest part for little kids is getting used to breathing, now my kids hate swimming without a mask and snorkel! We made it all the way out and back, taking about two hours. As the other reader suggests, inflate the BCD just a bit, or even more to rest upright in the water. Encourage them just to "hang" in the water to rest. Don't touch the bouys! They harbor stinging nematocysts. The best thing to do is stay relaxed, just flip the fins a bit, not swim hard and they should have lots of energy to make the whole trail. Even if you do not make it to the biggies, Disney has sunk all kinds of large jars into the sand. They are full of interesting fish that will show themselves if you just hover quietly above them.
Carla