I would absolutely let my kids tour the parks alone at that age (14-15 is around the first year of high school, so if they are responsible and can handle being alone without supervision then yes), but I would have a plan of action in case of emergency. Mostly in the event of something happening that would require evacuation. For example, if the park is evacuated and you can't contact each other, then go directly to the car, or whatever. Most people overlook things like that, but I think it's important to have an emergency action plan with kids who may panic in that kind of situation and not know what to do.