You'll need a notorized letter to allow any doctor/hospital to treat your grandchildren in a non-emergency situation.
Trust me. I got that phone call at 3 AM.
This happened 25 years ago, in the days before email. My MIL and FIL took my DD (then 2) to WDW, then to visit relatives in South Florida. Before the trip, DH and I signed a letter of consent in case of an emergency. Well, DD came down with the flu. My in-laws took DD to the emergency room. The doctors refused to treat DD since the letter of consent wasn't notorized. We had to send a telegram to the hospital giving permission to treat DD.