IMO she shouldn't have a cell phone. I have a hundred reasons why, and most of them to do with child development, pushing our children to grow up too fast, and buying into the idea that we have to have more, more, more.
As for Girl Scouts, I'm a GS leader, and it is the leader's job to stay with each and every girl until the responsible party has picked her up. So if the meeting ends early, the LEADER should be calling the parents, or just hanging out with the girls.
That being said........your daughter having a cell phone is your choice, and your MIL, (though I'm sure her heart is in the right place and she is truly only concerned for her granddaughter's welfare) should be minding her own business now that she's had her say. She should not keep harping on it.
I would just say, "Mom, I understand your concern about this situation, but this is what I have decided works best for us, and I need you to respect that. You did such a great job raising my husband, and I'm sure you remember having to make hard decisions like this. I appreciate your support as I make difficult decisions as a mom, too."
Good luck!
J.