Here are some recommendations. Keep in mind that the beaches on the eastern part of the island are directly on the Atlantic and so have bigger, more powerful waves. You may not want that for small children, but your older kids and teens would enjoy it.
Old Silver Beach: On Buzzard's Beach in Falmouth.
Beaches along Cape Cod National Seashore: CC Nat'l Seashore stretches from Chatham to parts of Orleans and Wellfleet. There are beautiful sand dunes and a wonderful shoreline.
Beaches on Martha's Vineyard (take a ferry from Woods Hole, Falmouth, or Yarmouth).
Surf Drive Beach: In Falmouth . . . small but uncrowded.
Nauset Light Beach: In Eastham . . . and see the lighthouse too! (On the Atlantic)
Woods Neck Beach: Falmouth . . . lots of little inlets and shells that will appeal to the younger kids.
Finally, here are some beaches that do not have lifeguards, so you may want to think twice about them if you have young children:
Breakwater, Crosby, Paines, and Robbins Hill beaches in Brewster
Lighthouse Beach in Chatham
Cooks Brooks and First Encounter in Eastham
Chapin Beach in Dennis