Knitette, love your quilt. What great outfits they are wearing.
Gramajo, you're bright colors and backing are perfect.
Wildyard.....have you considered ironing on a muslin interfacing? They make one-sided. You could stabilize it that way and add the rest of your blocks. Once the top and sandwich are together you could heavily quilt it, especially along the seams such as SID on both sides of the seams. I think that would hold it well. What do you think?