For big, fun waves and oceanside activities and fine, hard-packed, tan-colored sand that's great for biking or driving on, go to the Atlantic/east coast. Daytona has lots going on at the beach (including restaurants, shops, mini-golf, etc); Ormond or Flagler are quieter, Cocoa is near NASA.
For gentle, clear, warmer water in offseason, gorgeous sunsets and soft, white, powdery sand, go to the Gulf/west coast. Clearwater is easy to reach with oceanside activities (watersports, boating, etc) and has a decent public access beach. Some of the other areas mentioned are great too.
Which is best depends on what you all want to do and the ages of the people going. HTH!
don't want to intrude on your post but are there any good beaches to take children ages 11&8 snorkeling??
Don't know about this but you may want to consider visiting one of the state parks within an hour of WDW that has freshwater springs. When canoeing in these, it's like looking at an aquarium - crystal clear. Some allow you to swim with manatees. Just verify that the swimming area is near the headwaters and you'll avoid alligators, who don't like the cooler water anywhere near there.