Since you don't want to dismiss her struggles as too adult for her, I suggest you just tell her the truth. For example: Daughter, you are seven years old and you have an appropriate body for a 7 year old. As you grow older, your body will start to change and eventually, when you are a teenager, your body will look very much like mommy's body does now. If she requires more information, I would purchase a book like The American Girl Care and Keeping of You and go over some of the changes that occur with puberty (acne, breast development, hair growth), etc... I don't think it needs to be any more complicated then that. FWIW, I wouldn't label her prepubescent body as a "boy body" because she does not have the anatomically correct body parts to do that, so IMHO, that would be very confusing for her. I would call it a "little girl body".
