EmmyG---Having a grandmother in your life is such a blessing for children--and especially during times of change (as your family is currently experiencing). Gosh, I wish my mother had l ived close..I would have given her as m uch times as she wanted with my kids!!!!
Having said that....you and your mother need to be in agreement as to the guidelines....no missing days from school for discresionary "trips", for example. You may have to begin establishing boundaries of this nature with your mother. I am not saying to say it mean--quite the opposite. You might have to have several conversations with her for her to understand where you are coming from. I don't think not allowing her to see the children for long periods of time will benefit the children.
If she sees that she will have to compromise on her end of things in order to have access to her grandchildren.....hopefully, that will be motivation for her.
Emmy...the above is just a suggestion of my kind of approach. Of course, I don't know your mother. I'm just saying: Say what you mean;mean what you say;but, DON"T say it mean.....and don't let the kids suffer from being in the middle.
Best of l uck.......
dandylion