Away Wee Go offers a variety of wagons and probably has one to suit your needs.
Radio Flyer had fold down seats, seat belts, and cup holders making it an excellent choice for hauling the kiddos.
Step 2 Wagons have extra large wheels and long handle to help bring you gear to the beach. Some models do have seatbelts, and all models have removable sides to convert wagon into a low table or bench while at the beach or park.