How do I work through this?

We have a blended family with adult children ages 19, 20, 21 and 23.

So my step daughter is moving back home with her 2 young children and asked me to find a reliable babysitter because her grandma/my mother in-law(who runs a daycare) said she would only take them a few days a week. I said sure and posted on my Facebook that we were looking for a daycare. This post started something. The next thing you know I’m receiving message after message from mother in-law. At 1st they were just saying how hurt she was by it. I tried to explain that it was obviously a misunderstanding.
Then she decided to bring my children into it. Saying that it’s rude they don’t visit her on the holidays. They only came before because they got gifts and now that she doesn’t get them gifts they don’t come. She cuts them off her gift list at 18 (totally understand)… However, my adult children already have 5 other places they go to. They would come to grandma’s house but all they do is watch others open gifts. Is it wrong of them to skip out if they have other places that they go where they feel they are included in the festivities. Plus, how do I make them do something they don’t want to do? They are adults that do not live under my roof. How do I respond to my mother in-law? Do I talk with my kids about what she said? How do I get it across to grandma/mother in-law that they are adults now and I can’t make them come.
Help please, I do not like all this tension between us.

I would just tell her that you’re sorry she’s hurt, but they have multiple places they have to go. I would mention to them that her feelings are hurt by it, but ultimately it’s their decision if they want to go or not.