Since you are dealing with more than one hole, I wold find a appliqué shape that followed the theme for the shirts. I would iron on the appliqué shape with wonderunder and machine buttonhole stitch the shape after getting the interfacing on the back of the shirts. If she's a runner or cyclist, maybe an outline of a yellow traffic cone? School shirts could have the school initials or logo? Misc. shirt could have something generic like stars or hearts?