Are you certain that your inlaws wouldn't use a legal document like that to potentially get you in hot water with the Elders™, especially if you put in writing that you consent to your son receiving blood transfusions if deemed medically necessary?
Are you certain that your inlaws would even respect such instructions related to your son's medical care in the event it was medically necessary?
To be perfectly honest, I would NOT permit a child of mine to have unsupervised time alone with JW grandparents, unless I was a JW myself and agreed 100% with their teachings and doctrines. When you are "fading", Inactive™ or attempting to distance yourself from the JW religion, allowing an impressionable child to have unsupervised access with JW grandparents or other JW relatives who might feel they have a 'mission' to save them from your 'reckless' course of action (i.e., Leaving The Truth™) is asking for trouble.