Would it be okay to refuse to look after my grandchild so my daughter can go to school?
Why don't you want to look after your grandchild?
I want to enjoy my old age without any problem.
It is understandable that you don't want the responsibilities of caring for a child again. However, being there for your daughter and your grandchild will help ensure that your grandchild is safe. Instead of denying your grandchild the help she needs, you should support her until she can her way on her own.
I'm going to threaten to not look after my grandchild to give her an ultimatum.
The parents of your grandchild should take responsibility and need to step up. It is their duty to look after their kids, so let them know what you plan to do and tell them to hire a babysitter if they can't be there for their kid during the day.