We went to Chankanaab on our 12/7 cruise. I have a daughter that is 7 and a 12 year old. I would have preferred Mr. Sancho's or a different beach, but my SIL had been to Chankanaab last year and wanted to go back. In all honesty, we did enjoy our time there, even if it was not exactly what I was looking for in respects to my daughters' interests.
The lagoon/beach area would actually be great for a young one if they do not like waves. I did not find it to be crowded, it was not as tiny as I thought it would be and the children that were in it were spread out. At the deepest end I think it was about 3'. There are little tiny fish swimming around, and a couple large rocks (or coral formations? I don't know for sure) in various parts of the lagoon that children enjoyed hanging around and watching the fish dart back and forth into the little openings.
The grounds are beautiful, it was interesting to walk through the Mayan ruins, even if they are reproductions. We did not go into the museum nor see the Sea Lion show, but I did hear lots of laughter coming from the stadium where it was held.
We did snorkel, even my 7 year old for the first time. She did enjoy it, did not want to get out of the water for a long time. I have an intense fear of sharks, but I snorkeled for a bit too, and was glad to do it. It really is amazing! The water was not too wavy, and even staying close to shore (so the sharks could grab those people further out first

) I saw a lot of neat fish and coral.
The stairs can be slippery, but they aren't too bad. The hardest part was getting out - time it to the waves. Make sure the wave is pushing you as you step, do not step up as the wave is going out. It makes it a lot easier that way.
I did see some humongous Iguanas around the other lagoon (not the swimming one). They blend in so well, the only clue I had that they were there is that every so often, one would move to reposition itself on a branch. The stayed way up high in the trees, so I would not worry about them.
There were also little lizards running around, we saw them in the Mayan ruins area, did not see any near the water. My eldest loves all bugs and slimy-type things, so she was in heaven chasing the lizards and capturing one to take a picture of it (not to worry, she is very gentle and released it immediately afterwards, although she is still trying to convince me that we need to buy a lizard to go along with our 3 cats, fish, and 1 dog!).
One word of caution - make sure that if you go here, you do not pay for the 4 y.o. She should be free, but I think they are counting on people not knowing that.
Bathrooms are a problem - they had not running water in the sink, no toilet paper, and definitely no soap! We were there in the afternoon, so maybe these things were there earlier in the day, but they were not replenished. Take hand wipes and tissues if you will be using the bathroom!
All in all, this was a very nice place, something we would not see here in the States (at least, I don't think we would?). However, if you are looking for the whole "beach experience", with waves and excitement, this is probably not the place to go. But if you want peace and calm water for the little one, then do try this out.