To answer your question, a document pertaining to your child's health care decisions should be worded something like this:
My son/daughter, (insert name here), is temporarily visiting their grandparents, (insert names here), from (start date Day/Month/Year) to (end date Day/Month/Year).
In the event of a medical emergency, my child's well being and life are of the utmost importance. We can be reached at any time by cell phone at (###) ###-#### to be kept informed of my child's condition and to grant consent for any medical procedures that may be required.
My child's current medications are:
(medication name, dosage, taken x# times daily) [repeat for each medication your child uses]
My child is allergic to: (list allergies, especially medications like antibiotics, and include food allergies)
This document is in effect for the duration of (child's name) visit with (grandparents' names), (restate dates above).
Your printed name and signature
Your husband's printed name and signature
Dated this (date, month, year) at (your city of residence).
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Under no circumstances should you relinquish your decision making rights on behalf of your child to anyone else. In this day and age, where instant communication is possible via cellular phone, there is no reason whatsoever to give that right away to someone who might make a well-intentioned but poorly informed choice that you yourself would not make on your child's behalf.