Ouch, tough situation. You are really the only one who can decide whether the rewards are great enough for you to keep saying yes. (I'm a granny of a 7yo boy myself.)
As one of the world's great pushovers, I've had to learn how to say no, and also learn when I really want/need to say no. Two different learning curves!
Saying no is really the easy part, but probably will take a bit of practice, maybe in front of a mirror or one of your caring friends. Try this phrase out and see how empowering it feels: "No, that simply doesn't work for me." That one simple sentence gives all the information you need to convey, and in fact, if you start adding explanations, excuses or arguments, you'll only weaken your position. Try it; it works. If the other party argues, pleads, or wheedles, calmly repeat. "No, that does not work for me."
Deciding whether or when to say no sounds more complicated, in your case. Your request reveals that you some issues common to many women:
You see yourself primarily as a care-taker (at least in the eyes of others), and that is part of the "identity" you may need to work on to change your position.
You have the sense that you are needed by your grandchildren, both emotionally and practically, and that is probably significantly true. That might be very important to you, especially the emotional bond you have formed, and which is no doubt important to your grandkids, as well.
But you have established yourself as a pushover, a pattern that probably extends far back into your child(ren)'s life and your earlier marriage, and most likely into your own childhood. We work so hard for so long to meet other people's expectations that they seem to become part of us.
If I were asking your same questions, I would already be pretty cognizant that the current state of affairs is not working. I'd tell the parents that as of 1 week from now, they must find a regular sitter or daycare arrangement, I will no longer be available (except for "special" times of your choice), because this arrangement is not working for you.
People who are used to taking care of others without thanks get treated that way because they allow it. They allow it because of habit, because they were "trained" for it, and often because they are afraid nobody will like or love them if they don't. You will probably ruffled feathers by instituting a change. Family feelings may be rough for a few months or a few years. They already take you for granted, will annoyance be that much worse?
But be more caring to yourself; be consistent about staying in loving contact with your grandchildren, who already have a long-term relationship with you, and I'm guessing that some time in the future (maybe even very soon) will bring a realization by your family that you are really worth their care. Set that example, and don't launch a new generation who can't or don't appreciate their elders.