When my daughter became pregnant I said straight away I couldn't do regular childcare but would be happy to help out babysitting now and then and obviously any time in an emergency. She went on to have three kids but moved away. I hated saying it but I had to set ground rules. We are just back from looking after them for three days in school holidays which was a pleasure because it doesn't happen every week. I just think u need to stress that you no longer feel fit enough to look after them every week, but would do in an emergency, and set whatever times u feel up to doing.
Perhaps they even don't want to hurt your feelings by suggesting it's too much, and would be glad for you to do much less hours. Stick to whatever you're comfy with, make it clear it's because it's now to much for you, and give them time to make arrangements. Then your grandchildren will be a pleasure not a chore.
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