biteme, LET HER PLAY!
if you tell her you don't want her to, it's going to kill her confidence.
if you encourage her, however, think about the boost that's going to give her pride and confidence. how many girls can say they played football in junior high? ok, she will probably get laid out a few times. it happens.
you be there after every game to tell her how proud of her you are. encourage her the entire way.
if she wants to do this, you have to let her. don't say no to her. this might be a dream come true for her. let her have this dream.