If you think that they will not take much notice or make much effort then I suggest a little white lie, or you could actually get a doctors appointment and check if there is anything else that is causing you to feel so exhausted, but it sounds as though it is simply too much . So if you think you would find it easier to tell them that the doctor has told you that it is too much and you cannot do it any more. You could also say that if you dont rest more you will not be able to do any childcare at all.
Being generous , it could be that they dont see that you can do less as you get older, and if they have been used to your being very fit and active they still think of you in that way.
Or more likely, they are just being selfish and dont want to notice that you are doing too much as they will have to do something themselves.
My suggestion about the dr is so that they cannot then try and make you feel guilty or be pressured into continuing. To add to this I think it would be very good to give them a time, so perhaps easter or a week later when you will be away for a few days. Really do go away, even if it is just b/b for 3 nights and not very far away. Perhaps go with a friend or have chance to meet up with an old friend for a day during that time.
The point of that is , firstly you do need the rest, but it will also show them that this is now happening and is not going to change. However much you enjoy your grandchild, he is their child and they should be working out the best way to organise their lives, without expecting you to provide permanent care. Then when you are away, you could think about what you want to do, dont think about what they want. Just think if you would prefer to provide childcare on a set day or two, or only be available for some of the school holidays. Whatever you decide then you can present that to them when you go back. It is important that you just offer what you feel you can manage, and dont let them twist your arm to do more. You can always say that the doctor has said that you are not up to doing so much and if you continue you might need support yourself.
We all try and help our children of course, and with the cost of living things are difficult for many people. However I do think that many children think theyhave the right to expect help and are not grateful for what we do for them. In my case, I moved and lived abroad and travelled, so never expected any help with my family. I was grateful for any help I did get but it was not a right. We all muddled through in the best way we could , and made plans with friends and found ways of looking after our family and working. They try now to say how hard the mortgage etc is but I was in the middle of buying a house when the mortgage went up to 15% which they have never seen. So do take a break and hopefully get to a more practical and sensible arrangement. GOOD LUCK