This is so sad, jules, especially for such a living doll as your dd is!
Self esteem is such a shaky thing. I agree with Keli, it's sad but true that little girls need a "male" to convince them and Mommy's words sort of sound like an echo to them.
I remember telling my son when he was as young as 4 that girls need to be "validated" and you, my child, are the validaTOR. He's taken it and RAN with it.
He takes time to make sure that a little girl who does not feel "pretty" DOES feel pretty. He tells them that their shoes or clothes are nice, or notices their hair. He compliments them on their smiles and friendliness and does this regularly. The result, well... can I say the little girls in his school just LOVE him????
He does it for his teachers, too, and not just the young, pretty ones, but the older ones, too. Helps with grades!
I know what she's feeling, and 7 is not too young from what I've seen. In 2nd grade my son's classmate and her sister had their Mom dye their hair BLONDE because they wanted to be "pretty"!
It is sooooo sad.