Yeah, I think you made the mistake by allowing her to send her daughter in pj's.
Now that it is clearly an issue for you (which you figured it wouldn't have been!?), I'd tell DCM that it's just too much since DCG clearly doesn't want to follow directions and get ready in your care, so she'll need to arrive to you dressed and ready. Just explain that it takes up too much time in the morning and while you were trying to help, you feel it's making matters worse now as other children are arriving etc.