For small kids Mayflower Beach in Dennis is great. Lots of sand bars, the tide goes way out. However, not too much beach when the tide is full, people are almost sitting in your lap.
In North Truro, Head of the Meadow is great and so is Cold Storage Beach across the Bay on Highland Road. It's a shallow beach at low tide.
He that would make his own liberty secure, must guard even his enemy from oppression; for if he violates this duty, he establishes a precedent that will reach to himself.
Thomas Paine