I based my recommendation on the statement that they wanted to let them go on their own to shop, snorkel, etc. The peninsula are pretty upscale condos and not someplace that you are going to find much teen activity. If you want to let them go all over by themselves, then they have more to see and do on the south side. The down side with that is, as I said, no beach chairs, bathrooms, shade or
activities without paying to sit and consume food/drinks in return for the use of the chairs.
What oneistwo said about the hotel zone having the best beaches is true, the beach on the south side by Los Muertos (where all the beach
restaurants are) is small and crowded, certainly not the best beach to swim.
I guess you can say there are good and bad about all three places the OP named, deciding what is most important on their list will ultimately help them make a choice.