I vote for Coki Beach. We ( kids, 4 and 11 included) had a great time there last summer. It is a beautiful beach and there is an aquarium there with lockers and bathrooms to use. We made a day of it there and did not regret it at all. Bring a mask and snorkel if you have it and some Milkbone-like dog biscuits ( the bigger the better) to feed the fish while at the beach. Or you can rent the gear and buys the biscuits, 5 for a dollar, at the beach. There is also a bar on the beach if you chose to indulge just a bit while sitting on one of their beach chairs, under one of their umbrellas... all available for rent. Check previous threads about Coki Beach for the name of the aquarium because we were able to print a coupon for the aquarium directly from their website. Their name escapes me right now.
Have a great time.
Michelle