Yikes. That's a hard situation.
I agree with others to let your children know that you and your husband need a break too and discuss having a weekend free a month. Surely they can find someone else to watch the kids during that time.
It is hard with young kids, I have a three year old and a 6 month old. My mother usually keeps them one night a week. We'll drop them off around 7pm or so and pick them up at 9 am. This is mostly so we can get uninterrupted sleep. However, I make it perfectly clear to my mother she under no obligation to do so. In addition, we've begun to discuss what to do when my son starts public school. She mentioned keeping him after school but I don't want my son to become her obligation.
Sorry, I got off on a tangent.