Just be honest. Seven year old girls aren't stupid. "Most comics are made for older guys and teen age boys that don't have girlfriends. So they try to make them feel better about it by making comics with half-naked big boobed women. Sad perhaps, but such is life." Then proceed to show her the many comics she would like, from Archie to Oz, there are still plenty of options. The new Ms. Marvel is a particular stand out.

Trying to convey this as some sort of social ailment is rather silly. It's simply human nature. And unless you think we should all become asexual, it isn't going to change.