I get so mad when anyone be it Grandparent/grown child or even the grandchild themselves, feel they can dictate the terms of those roles. Its all about a mutually agreeable relationship. There are no hard and fast rules, no 'job descriptions'. Sometimes a Grandparent feels they are not called upon enough and sometimes the grown-child seems to feel threatened and defensive. Sometimes its more like this OP where the grown-child sees their own parent as a free childcare provider and open ended support system.
Relationships are such nebulous things, they need careful nurturing and a whole lot of respect (that just might need to be earned).
Sunnym, you can only do what you can and no more. It does not stop you loving your children and grandchildren so do not feel guilty. Offer what you can and then just reiterate your health limitations, smile and leave them to it. All the best.