Best solution is to start with a detergent soak followed by pressure washing. If that isn't sufficient, then a good next step would be to blast it. Soda blasting would be an excellent choice as would glass bead.
Another thing you can do that would look great is to take your random orbital sander and get a brown 3M pad that fits onto it, and scrub it using that. You will get a very pleasing matte swirl pattern, way better than using a flap wheel or other heavy abrasive thing like that.
Or, you could bribe your kid with a new video game and turn him loose with some no. 4 steel wool .. :-)