Hey Hapa,
Good question, I don't recall seeing a sandy beach in Potato, it is pretty much all dikes and marsh islands. In Bedroom two there is a dock with a small club house of some kind, I have never seen anyone there. You can dinghy around the corner to the Lighthouse, looks like there may be room for the kids to run around. I took a look at Google Earth to see if I could find something but the only sand I could see is Peter's favorite spot on Franks. Someone else may know of one.