Have you quilted the sashing right up to the block seam? If not, could you carefully snip the thread line along the t-shirt square only? Then take the old square to use as an exact template for the new one, cut it out and slip the 1/4 in. seam allowance under the sashing edge. Pin it all around and carefully hand stitch the seam lines all around again to the sashing. I would use the invisable ladder stitch to do this. Just an idea. What a great Mom you are to go above and beyond the call of duty.