Favorite family beach vacations for us throughout the years have been mostly on the east coast since that is where we've always been based.
Some of our favorites include Montauk on the East End of Long Island, NYS, Jershey Shore (in the past, Seaside, but now, Cape May), Virginia Beach, South Florida and the Florida Keys. I missed a family beach trip to Myrtle Beach, SC & Outer Banks, NC, which everyone also loved, wish I could have made those! There is something special about the Florida Keys in that being there almost feels like stepping into another time, more old fashioned (though modern family friendly resorts abound!) and "old timers" with a different vibe than say, when visiting beaches in Florida proper.
The Keys are also a bit of a first sample of the Caribbean, where fun times were had on Paradise Island, the Caymans, St. Barth's and caribbean Mexican beaches.
It's amazing how fast a great day on the beach can pass by when everyone is having fun at a wonderful beach locale. There is always so much to do in the towns but the sand & sea are peaceful, carefree & fun for everyone, all ages, anytime!
I only listed a some, but the possibilities are endless and the fun is limitless!