From what we saw on our cruise, I guess
DCL figures the average child is potty trained around 3. So, when they spot check the pools(although there was always someone at the Mickey Pool) they approached children who were obviously under three because of looks, they were with a parent, had a diaper on, or were using some sort of float.
When the Mickey pool was not crowded, we asked if your 16 month old son could play in the blue section of the pool, this is where the water shoots up at a given times of the day, some of the "pool watchers" allowed us, but there was one who always refused.
Honestly, the Mickey pool gets soooo crowded and becomes so foggy, I wouldn't want my son in it, especially with the illness going around.
If you are traveling with a nonpotty trained child, here is what we did. We trapped our son using the chairs, table, and railing as a barrier. He had a nice spot in the center, we all sat around him, and he played with his sand toys and a bucket which we kept filling up with water. He had a blast! We did this by the Goofy pool.