How to handle this situation?

Yesterday, my MIL had my 5 year old daughter over and my nephew and niece(4&6) they went for a swim and when it was time to go. the girls didn’t want to get out and were screaming bloody murder. My MIL brought LO home and she was emotionally exhausted, and started to cry. Told me about how they won’t listen to her. I told her what she already knows, she can call us ASAP if she can’t handle the situation. Even though before my daughter left and warned her to listen to grandma and if she says it’s time to go, no discussion. So now the consequence is she’s not allowed to go over unless we are there too. My MIL doesn’t think it’s a good idea because she’s afraid her grandkids don’t want to be around her if she gets mean or sets these types of boundaries. Don’t know how to win this one. Any suggestions?

Edit- Grandma does use transition warnings, also uses a timer. And they still refuse.