This is just my opinion but we like the beach at Amelia Island and the one at
Tybee far better than the ones at
St. Simons Island and
Jekyll Island, though St Simons has a nice park called the Neptune Park Fun Zone that you'd really enjoy with the family.
also, if you stay a few days at Tybee you can take day trip to Savannah where you can enjoy a bit of shopping and picnicking in the parks.