I had my husband's fellow Marines write letters to him when he retired and they had a VERY difficult time doing it until I suggested they write about him instead of to him. It worked great. They all churned out really nice stuff!
So, try writing your rough draft as if you are writing about your daughter (use her name and the pronoun "she"). Write as though you are speaking to her teacher about how proud you are of her. Don't worry about it sounding perfect, just get your ideas out (write as though you are speaking).
After you get all you ideas out, then you can go back and change the pronouns to "you" and polish it up to sound "perfect". I think you'll feel better about it if you do it that way than if someone else gives you the words to use.
What a sweet idea, by the way.