The interfacing is the best way to go, I agree, but from now on keep your daughter out of the fabric dept--or just totally out of Wal-mart and Joann. LOL
I've used a lot of Joann fabric, but you have to be fussy, and this might be a good time to start teaching your daughter what is good quality and what isn't.
This time you are just plain stuck. She will see it through her eyes, and won't see the troubles. Just the pretty print that she fell in love with. She won't see all the extra work that you will.
And we are here 24/7 for you.
Hang in there Mom.