My advice would be to stay away from the topic of WHY there is a divorce. (It's rarely only one person's doing, and even when one spouse has had seriously objectionable behavior, the other spouse usually did things that hurt the other person and led him/her to make the wrong choices, even if it was unintentionally.) After awhile, if the two of you build up trust with each other, she may tell you things that are helpful for understanding her decision. Also, if you stay kind and approachable, it could contribute to any reconciliation possibilities there may be in the future. Don't let awkwardness and ill will between the two of you become another obstacle that stands in the way of possible reconciliation.
I'd recommend writing a note saying something about being sad that the marriage is ending, but that you intend to consider her family forever. As the mother of your grandchildren, she holds a place of honor in your heart. (Put away all judgments at this point, and do not even hint at them.) Let her know that you hope she feels the same way. A note allows you to choose just the right words, and enables her to re-read it to shore up her nerve when she's ready to take you up on it.
In this note, invite her to call you to ask to keep the grandchildren as often--or more often than before. Mention that you recognize she may need some extra support, now that she'll be the lone the adult in the household, and offer to give that support in the form of spending time with the grandchildren. Then, if she doesn't call soon, call her yourself, and without mentioning the note, ask if you may take the kids to a movie (or some other specific activity) and when would be a good time that might also be helpful to her?
If she has the gall to ask you to babysit while she goes on dates in the future, look at it as a way to keep your son and her previous marriage in her mind as she dates! I guarantee, it will cross her mind several times during her date.
Do the same for your son. If he's like most men, he's probably never been fully responsible for looking after the children by himself for an extended period of time. He may welcome your help.
Do understand that for both your son and daughter-in-law, any time that the children spend with other people is time away from the parents. Now, each parent will have to share their time with the kids, reducing their time with kids by half or more.
Be very careful to never speak badly about your daughter-in-law when your grandchildren can hear. Children take that in and often have thoughts like, if my mother is a loser, what does that make me (her children)? Might Mommy also leave me if she gets mad at me?
Find some resources for parents and grandparents dealing with divorce and read all that you can.
I wish you the best, as you travel down this difficult road. Be a blessing.