The reality is that no solicitor will draft a disclaimer for you because one does not exist to cover the situation. If you do something negligent and because of that negligence someone is injured then you can be sued. Even where the negligence is questionable, you can still be sued- you might not be held liable, but do you want to face a few years of worry and legal bills before a court verdict?
Forgive my ignorence, but I dont know how any of us could be negligent. We are not adhering to any standards, and all of the parents are on the spot. If any of the helpers are being "negligent", they will be doing so under the glare of all the parents.