My 12-year-old niece who has a round face currently has a layered shoulder length haircut and it looks really nice on her. The thing with long layers is that whether you straighten your hair or let it dry wavy it looks good either way! Also, layers will take weight out of her hair and make it less prone to tangles and overall much easier to manage. For a round face, I don't think you should make any layers shorter than where her chin is as it will probably make her face look rounder. You probably don't want to go too short all at once either, especially if she really likes her hair. When I was 13, I decided to get my long hair cut into a short chin length bob and I really hated it because it made my round face look bigger and it made me feel disproportionate. I wasn't very happy!
She could go with this without a fringe:
or maybe a little bit shorter like this:
This is basically the cut that my niece has right now:
She could go with this without a fringe:
or maybe a little bit shorter like this:
This is basically the cut that my niece has right now: