With kids that age you want to be on the south facing beaches. We spent years in Harwichport from the time when our kids were 2-3 years old on up to their teenage years at which time they want the outer beach waves. Harwichport is close to Chatham for shopping, Cape Cod Baseball League games and nice calm beaches (although in June the wind will blow SW)
Try here
Commonwealth Associates and ask for Neal or Patty they will help you find a place.