When people decide to have children, they need to consider longer term arrangements for their care. In many ways, it was simpler when ours were young. Childcare was pretty much non existent and so we stayed at home, perhaps worked when husbands/partners were home and tightened our belts.
Childcare is much more prevalent now, but many find it difficult to afford. But this is one of the considerations couples should think about beforehand. Yes, it's hard looking after babies, so why would it be less tiring and hard for a grandparent, who, with the best will in the world, is getting older, particularly if still working outside the home.
I live too far away to have my grandchildren regularly, but when I do, although they are older now, the responsibility is tiring, as well as the fact that I'm thirteen years older. I think your son and DiL are unreasonable OP. They have the children, they look after them. No one I'm sure, minds helping out, but a regular commitment is a different thing altogether and, in my experience, can get to the point very quickly where it's another day and another day and an expectation that you will always be available. And it is blackmail, of a sort.