Goodness--you were working with what she gave you. And the whole idea behind this is not how bright it is, but how comforting it is. Right?
I'd definitely charge more if she wants it redone. You did as you were asked--and if she needs a change, it should be on her bill.
What if you appliqued hearts--bright ones--in the white squares?
Maybe quilt it with a bright thread?
I'm sorry you didn't get raving reviews--I think you did great.