When your mom's emotions calm and she thinks about the way you handled the situation she is going to feel even worse. You kept your head, spoke in a calm and loving way, and left the door open. Good for you. You are making the best of a bad situation and at least you can have the comfort of knowing that you did not say anything you will later regret. Give mom time to think about things.
I was thinking that perhaps you could arrange to meet your mom halfway to let her see your children. She could pick them up at a halfway point and then later that same day or the next return them. That way the drive would not be too far for either of you. The children need a Grandma who cares about them even if the circumstances are less than ideal.