Improve weatherworn, stained rubber saddle on outdoor playground toy
Did you try using some Armor All or other silicone based treatment, typically sold to dress up tires and car dashboards? It will eventually rub off, but the silicone is relatively inert and harmless. There is a petroleum based vehicle suspension with it to make it spray-able that will evaporate relatively fast, so just keep the kids off of it for an hour or so. You will also have to re-apply it periodically.