I tried to convince my daughter to use some fabric she did not particularly like and now wish I had listened to her.
I like the quilt, she does too, but it caused some strife I could have done without.
Let her pick and go with it.
If you need to add fabric to finish the quilt, pick within her range of colors.
I made a rather monochromatic quilt and when I asked a shop person for help choosing border fabrics, they "complained" it was too monochromatic and selected fabrics I could not go with. I picked one of her selections and have been sorry ever since.
You have to listen to yourself and in this case, to your granddaughter.
Let it go.