Since it's a wall hanging, I doubt anyone will see the back so I'd just leave it as is and finish the way you began. Obviously I believe that our mistakes are meant to be "learning curves" also if your niece thinks she doesn't want the white thread on the back she can use a fabric paint/marker and trace the stitches, I did that to the border of one of my quilts because I didn't think the quilting showed enough. Just my opinion.