You have 2 options that will be effective.
1. As stated by TargetIndy, The landscape rake on a tractor will get MOST of the sticks and thorns, but not all of them unless your forest is perfectly level. Then go back in and rake up the areas you missed.
2. Go to your local BLM, or Forest Service office and ask about getting a burn permit to do a controlled burn of the area. If you have noxious weeds it will make it even easier to get one. Use a tractor, skid steer, or bulldozer to cut a fire line around the perimeter of the field and it will also provide a clear boundary of your play zone. Burn the area, and you will not only have no thorns, you will end up with more diverse ground cover with more vigorous growth. The BLM or forest service might even send a crew out to help you. hope this might be of some help.
Last edited by Misha"mole"yelich : 07-17-2012 at 04:10 PM.