While there is no fast and loose answer on this, you should at least look to contact a personal injury lawyer. If the park you injured yourself playing on is a public place and is supposed to be maintained to a high standard, you could be in with a shout. If your injury happened on a private field where you paid to use the premises, you more than likely have some case.
I would recommend you find out who your local personal injury expert would be. My local is Abramson, Smith, Waldsmith LLP, and I would hire them if I ran into any of the problems that you are facing. The sooner you act, the more likely it is that you can get compensation. I have usually found that the longer you wait, the less likely it is that your claim will be taken seriously.