Grandparents as carers and being called by the wrong name

Being a grandparent who regularly cares for grandkids for whole days, or even multiple days a week, comes with lots of joyful moments but can also be incredibly tiring. What's the best way to manage it and be able to say no when it becomes too much? Also, why is it so infuriating to be called the wrong name and how can you correct people without seeming rude?