I want to add, make sure it says "medical decisions" and not "emergency medical decisions". Had my cousin with us once, and she was having some medical issues, not life or death by any means, but medical issues. We weren't able to get her seen, as we weren't her parents and the medical release we had was just for "emergency medical". We were informed at that time that typically doctors and hospitals will take a child in for a "true emergency" situation without parental approval, and will stabilize the child, but nothing further. Things like ear infections, strep throat, swimmers ear, etc, are not "true emergencies".
I've had friends have issues with traveling with young children before across state lines particularly in planes. And, I've also heard about problems when you do have a medical release, but it doesn't say that you have the ability to go across state lines. I've heard the issue being that the release is only good for that state if it doesn't say otherwise.