School-age students often use full-foot rubber fins to support their swim workouts without risking muscle strains. The longer bladed versions can be deployed for underwater swimming too. They are the norm for diving in Japan and were standard everywhere up to the 1970s. When I snorkel in the cold waters of the North Sea near where I live in North East England, these are the kinds of fins I have used for over fifty years.
You might consider, for example, Scubapro's Full Foot Fin:
http://www.scubapro.com/en-US/USA/snorkeling/products/rubber-full-foot-fin.aspx
It comes in a good range of sizes. A similar fin is the Head Corda:
which can be had from Amazon at
Amazon.com : Head Corda Rubber Full Foot Fin : Sports & Outdoors
There are many other fins of this type available and they can be so comfortable that it's easy to forget you are wearing them. And some all-rubber models, e.g. the Aqualung Sea Lion fins
used as resort fins in Hawaii and elsewhere, have very soft foot pockets and blades. It all depends on the blend, which can be stiff as with Jet Fins, or soft, as in the case of many workout/snorkelling fins. Unlike fins made from synthetics, they're also pretty indestructible.
For cold water, get a pair a size or two bigger than usual to accommodate neoprene socks. Try them on with whatever your 12-year-old is planning to wear with them to check fit.
An additional recommendation comes from one Pete Atkinson, an Australian underwater photographer, here with his rubber fins:
Read his story and the reasons for his fin preference at
My favourite kit - Pete Atkinson
