I would give the lady the dimensions of the quilt & border and let her get what she thinks she needs. It takes the burden off of you and puts it on her.
John Flynn has the system for saving fabric by cutting the piece diagonally. You have to calculate how much fabric by using a formula. What I did was subtact the usable width of fabric from the width of the quilt. Example (64 + 4)-40=28) 4" inches alows the overhang for the backing, then I took a piec of fabric (any fabric) and marked a diagonal line, measure from inside the selvege to the line and move down until you have a measurement of 24" (from the diagonal line to selvege). That measurement you add to the length of your quilt 68+ (measurement) and that is the amount of fabric you need to back the quilt. This allows 2" overhang around each side. I always buy at buy about 1/3 of a yard more for fudge factor and the extra overhang of the back. Now at some point (if the quilt is too wide) it will not save you fabric and you have to buy 2 times the length or what ever.
Since these kids are just learning, I think the best thing is to go with twice the length and be done with it.