Posture is often involuntary and the lowest energy position that our bodies end up in. This is why, regardless how many times you tell her, she will not maintain the better posture until she strengthens the muscles in her core. A great exercise to start with is called the bird dog exercise. An example of it can be found here. https://www.youtube.com/watch?v=wgOuR7YrwtM
Let me know if you have further questions.
Therapist for muscle and joint imbalance. She may be a candidate for postural retraining therapy.
Ask her if sitting correctly causes discomfort and/or pain and makes her back feel tired. Also, have her stand in front of you with her back to you looking straight ahead in a relaxed position. Look to see if her head is tilted to one side and if one shoulder is higher than the other. Then put your index fingers on each side at the top of the pelvis and see if one is higher than the other. Have her stand sideways and see if her head is projected forward over her chest. If any of what I have advised is present a chiropractor could help. In my experience children respond quickly to chiropractic care.