How do you tell your children whom you dearly love that you have raised your family and are tired of raising grandchildren. I have been doing it for 11 years now,for free, just because I care and love ...
That's a tough one.
I also babysit (for free) but I'm a relatively new grandma with just one grandchild (18 months old). I'm not at the point which you describe, but I can certainly understand your sentiments!
Perhaps you can start small - inform your kids that you will only be available 4 days per week. (Then over time reduce the days you're available until you've got it down to once in a blue moon.)
They may ask why, but I don't think you're obligated to provide them with a particular answer - but if they DO ask, you could always tell them the truth!
You've been babysitting for the past 11 years and you'd like some time to yourself to do the things you've earned the right to do. Period, no discussion.
P.S. They wouldn't necessarily have to pay a sitter. Some schools still have free after-school programs or the parents could get together and form an exchange co-op with a group of other parents who have kids with similar ages and take turns providing day care for each other.