When planning a trip to Florida for the family, the first destination to come to mind is most likely not going to be Jacksonville. I imagine Orlando is at the top of the list. Competing with Mickey and all his friends is not easy, and Jacksonville has nothing quite like Disney, but that's not all bad.
I lived in Jacksonville for a couple years and our favorite activity with the kids was going to the beach. Children of a certain age can spend endless hours at the beach and never get restless or bored. I know there are beaches all over Florida, but Jacksonville beaches in general are quieter, with not so many partying tourists and the beaches are family friendly.