No, you’re def not being unreasonable. My husband and I looked after our first grandson, who’s now 9, for one day a week. We were in our early 60s then and it was manageable for 2 of us. Now we’re both in our early 70s and look after our second grandson (14 months old) for one day a week. We can def feel the difference! We’re both shattered by the end of the day - and that’s sharing the care. I couldn’t do it on my own. I’d struggle to carry him for a start, especially in and out of the car. If your son and his wife are well paid they can get childcare, or as someone else mentioned, there may be some entitlement to free care. They shouldn’t assume that you can do it.
When our 2 children were born, my parents both worked full time and my in-laws were older and lived further away, so I just gave up working and we had to manage. Perhaps that should also be considered.
Talking about wealth: what happens if the government took on the mortgage debt?

