Are you certain she "can't" memorize the staff? Maybe, at 7, she's just not ready. Sometimes this kind of stuff won't come to a person for a long time - then suddenly one day it can "click" and the person can't remember what the fuss was. I had difficulty (5th grade) memorizing multiplication tables - couldn't do it. My classmates did, I just couldn't do the instant recall as fast or easily as they did. I worked out an alternate "count and add" approach that let me calculate the answers in my head fairly quickly. I passed the class and the teacher never suspected. Years later as an adult in my twenties, something went "click" and I now have no difficulty - basic usage got those tables into my head.
For this reason I would suggest letting go of the theory - still try a little reading every practice, but don't work it too hard. Try showing how the notes are related as intervals. Show how starting at one line and going to the next space means moving over one key or going from one line to the next line means moving up or down two keys. If she looks at those dots as a pattern of movement up/down whatever instrument, maybe something will click...
In all likelihood, repetition and time will do the trick. After all, she's only seven.