You can remove the stain from the gravel the same way you clean the driveway, with muratic acid sprayed from a pump sprayer. Im not sure of your budget constraints on this, because the dimensions of the playground is not given.
Have you tried muriatic acid on a small test patch. Be care full the stuff is corrosive and the vapor is harmful if inhaled. Always keep fresh water handy when handling the stuff
The best place to start for car cleaning products is microfiber. From scrubbing sponges to suede glass cloths to polishing mitts, many of the best vehicle cleaning and auto detailing tools are made...
If your painted cabinets can use a good cleaning, I have some advice for you.
1. Empty the cabinets.
2. Use the soft brush attachment on your vacuum cleaner and carefully vacuum each cabinet...