I would stop offering help with childcare right now! Your own health and well-being must take precedence!
When I had my own children in 1991 and 1995, I was in a Senior Nursing position and had no option but to return to work, as I earned much more than my husband did. I would have loved to work part-time but that option wasn't available to me back then. My own mother was ill and by the time my son was born, it was terminal. My mother in law was a lot older than my mum and had early stage dementia so had enough problems looking after herself! That meant we had to pay for childcare, and my feelings are that it is unfair for couples to look to grandparents who themselves are often in poor health to provide this, often free of charge!
I myself am now a grandparent and there is no way on this earth that I will be providing childcare or even babysitting! I had to retire when I was just 54, nearly 10 years ago now, and I have very severe osteoarthritis and osteoporosis, with several fractures. This has affected my mental health significantly and I suffer from severe depression. My daughter treats me badly and tells me to pull myself together, so there's no way I would put myself out for her! Thankfully, I live over 50 miles away from her, which enables me to keep my distance!
Your son and daughter in law sound most unappreciative of the help you provide! I understand that you want to see your grandchildren, but not to your own personal detriment! Tell them you are too unwell to provide childcare, and leave them to get on with it - it's their problem, not yours!