I'm sure that many grandparents care for their grandchildren in such circumstances without any official authority, but if it will ease your concern, I'm sure you could write a simple letter of authority saying "In my temporary absence I authorise Mrs xxxxxx, my daughter xxx's grandmother, to take whatever action is appropriate for my child's physical and medical wellbeing.
Signed: yyyyyy
It might help your mother in law if you give her a separate list of any illnesses, vaccinations, alergies you child has had in case medical attention is necessary. And enjoy your holiday. Your daughter will probably enjoy spending time with her grandmother.