Of course a grandparent can tell children off! What on earth is wrong with that?
I often have my grandchildren for hours at a time, indeed occasionally for days at a time and regularly overnight. Am I supposed to allow them to act in an rude or unruly manner.
No way! It takes a village to rear a child as the old saying goes.
HOWEVER because I've always been able/allowed to discipline if necessary, I actually find I very rarely have to, because my grandchildren have been raised by their parents, and us, to be well mannered (they are 6,7,8 &11) and no, they're not little robots, but lovely & lively and a delight to have around.