My child is going on vacation with his grandparents. How do I give her authority to sign for medical purposes?

Write a consent. In most states, you may be able to just write a statement saying that she has permission to get medical care for the child if needed. You may or may not need to have it notarized. Also, list any medication allergies, the names of any medicines your child takes, and any major medical problems. This can help if your child needs to be seen by a doctor while on vacation.