Raffles. I'm a youth worker and so I do all sorts of activities and such both inside a building and outside with the ASBO generation and so I've had quite a bit of experience from what you are looking at. If you need any help and such do PM, I might be able to pass you some helpful documents/policies etc.
An easy way is to get the parents to sign a disclaimer saying that they understand the sport is dangerous but understand taht you cannot be held responsible for accidents as long as they are following safey guidelines etc. We've taken them paintballing, abseiling, motorbiking and many other risky activities. We do have liabilty insurance though as an organisation but it might be a little over the top if you are doing it for no profit/gain.
CRB's are more for the benefit of the parents so that they know you are kosher. Best thing I can say is get to know the parents as much as possible, get them to an informal sit down/talk with refreshments etc and talk the stuff through. That way they know you and, should anything happen, be less likely to stich you up.