What a great time to hit the scrap basket and pick up your tonal material!
Those 4" squares never looked so good! You could try making a medallion quilt that has the rainbow in the center. Why not put multi like shades together in strips and then cut your arc from the colors. If you fuse or machine applique each color group onto muslin you can shape the rainbow to whatever size you want. If you use multiple patterned fabrics in the rainbow colors....you could make several different rainbow squares. Draw it onto muslin and then sew it down. :D oooohhh the possibilities are endless. Have fun. Ask her for her ideas...does she have a quilt building in her mind... could she draw a picture of what she is thinking? Would she want to help you? what a great way to get her into this wonderful arena. So lucky!