If you get several blocks done and decide not to do anymore, you could always applique them onto the black background. I would not cut all the black fabric up yet. Try this - take several sample pieces of fabric, cut your diamonds out, then cut card stock (or cards sticking in the magazines) and cut three diamonds out of the cards. Cut them 1/4" shorter on all sides. Use these for English piecing. Take the fabric and smooth it over the diamond card stock, fold it over all the edges, and baste it on with large stitches. After you have 3 done, lay them together to form the cube, and whip stitch them together. This is the English piecing method but it will give you another idea as to how to proceed. Both of your kids could do this easily. Some people do it this way to get them exact then applique the finished cube onto the background. Good luck in your quilting. Teach those children as you go too, because they grow up to teach others too.