All doors or gates shall be of such size as to completely fill any opening in the fence, wall or other structure and shall be equipped with self-closing and self-latching devices capable of keeping such gate or door securely closed, which closing or latching devices shall be located not less than four feet above grade or otherwise be inaccessible from the outside to small children. In lieu of such self-closing and self-latching devices, said doors and gates may be equipped with locks which shall be kept locked at all times when said pool is not in actual use.