Typcially, the best beaches for younger swimmers are on the west-facing side of the Cape, which fronts the more sheletered waters of Cape Cod Bay. Cape Cod's Atlantic-facing shoreline experiences much bigger surf, so not ideal if you have younger kids. One of my favorite beaches for kids is Skaket Beach near Orleans, which has big tidepools and shallow water that is perfect for toddlers who want to roam and splash around. In Chatham, you might try Cockle Cove Beach, which fronts the sheltered waters of Nantucket Sound. Hope this helps!