I strongly suggest that you set boundaries as grandparents. As an example, my sister told her son/DIL that she would take the grands every other Friday for the day. She would keep them 1 night. If the kids wanted it to be Saturday night, no she would not keep them Friday and Saturday. The kids had to drive the GKids over and pick them up.
This relationship has worked very well for 15 years now. She still takes them as requested. She offers to take them on special travel trips. She offers to take them to visit other family members out of state.
As the Grandma, she drives this bus.
I don't have kids/have not a clue what you're going through. I did listen to my sister set boundaries and she thinks it is the smartest thing she's ever done.
You're not retired to become daycare are you? And you sound very stressed out about not having a life. Don't be their free ride. You've earned your retirement. It belongs to you.