I'd think just write up something very general. "We, (mom's name) & (dad's name) hereby give permission to (grandparents names) to make all necessary decisions & take any action necessary in the event of a medical emergency where the parents cannot be reached."
Definitely can use some improvement, but I don't think it needs to be too legalese sounding. You have no trust issue that they'd misuse it, right? It sounds like they just want to cover themselves in case he needs any kind of medical care & the authorities (dr, hospital, etc.) say - 'well you're not the parent or legal guardian, so we can't take your word for it.'
Hope this helped & good luck with your adoption trip.