I'd suggest you get a wheeled underseat bag for her, as her suitcase. Then her book bag can have her entertainment, etc. If there is any overflow, which is doubtful considering her size, it could go in an adult's bag, but her clothes are small, so it shouldn't be a problem.
I can pack everything I need in an underseat wheeled bag for a 4 day trip, which means I could go any length of time if I wanted to do laundry, etc. all.the.time. However, I don't and I enjoy my luxuries, so my bag is larger, but so are my clothes. Sigh. If she is like my daughter, who tended to get dirty in the blink of an eye, she will need more changes than an adult, but with packing cubes you'd be surprised how much fits in those wheeled bags for a 7 year old, and it will be so easy for her to maneuver on her own so she'll learn how to watch and handle her own bags, and feel quite grown up. IMO