I would do a tongue scraper (or brush her tongue well) plus find a more kid-friendly mouthwash (does she actually like mint well enough to use enough mouthwash? Many kids hate minty mouthwashes), but if there's no improvement after a week, please get her to the dentist ASAP. I have heard of the tonsil stones but honestly I can't see popping something on a young kid's tonsil myself unless you know your child is going to hold still and not get a bit freaked out. I'd want a dentist to check to see if tonsil stones, or flossing issues, are behind this.
I want to add: If it's not tonsil stones and the things like cleaning and rinsing don't help, please see her pediatrician -- don't let it wait too long. Diabetes has symptoms that look like other things and one symptom can be foul breath -- Google "bad breath and diabetes." Even if you have no history of diabetes in the family, a child still can have it; I know several families with diabetic kids where there is no other diabetes in the family. If it's not diabetes, there are other medical issues that can cause bad breath. Most likely this is something simple like tonsil stones or just a body chemistry that causes bad breath -- but do not wait and wait if it continues; do see the doctor to rule out other medical issues.