My child is TEN, a very mature reader, has read all the books more than once, and was only just this weekend allowed finally to see the fourth movie ("Goblet of Fire"). We have kept her to the first three movies until now because of the intensity of the imagery and the increasing (though essential for plots) violence. I will not let her see anything beyond the fourth film for now -- despite the fact all her buddies are seeing it -- because it's too much and because (very important to me) I want her own imagination to be where these characters live, without her mind just filling everything in with the moviemakers' images.
Stick to your gut instinct and do not let your child of seven see this latest film. First it will overwhelm and on some levels even bore her, especially as she is too young for the detailed plot. And more importantly it is too intense and violent. You don't say in your post whether you have read any or all of the books or seen any of the other movies yourself. If you have not, be aware, this film is the final installment and culminates in a very lengthy battle that kills several longtime characters. If you have and know that -- then you are right in your instinct that this is totally inappropriate for a seven-year-old. Seven is even too young for the books, though many parents let kids this young "read" them or see all the films.
I've been at several earlier Potter films where parents had brought kids who were clearly four, five, six, seven-ish, and the kids inevitably were either bored silly during the talky parts and/or scared stiff (sometimes crying) in the scary parts. Stick to the ratings system and wait until your child is much older and you can truly enjoy these films together!