Updated and added: I just wanted to throw this out there since there have been a few responses talking about the effectiveness of these consent forms. I realize things can change and rules may differ, etc. but last spring '09 a neighbor was watching her niece and nephew while their parents were on a cruise. She called 911 when the little girl fell out of a tree. The computer printed form similar to the ones people have recommended, signed and dated by both parents, was enough consent to get the girl an ambulance ride, emergency admission, x-rays and medical care.
I would add.... some wording along the lines of 'In the event that we can't be reached..." That way you are saying if you can't be contacted first then whoever you appoint then has your autho to decide medical treatment. I would also add my childs height and weight to this info. This is good for whomever is caring for your children to have. If you have ever had to call 911, depending on the emergency this can be important info.