Don't know the severity but look up what lordosis is and keep that in mind for your daughter. It's caused by weak front abdominal muscles and tight lower back muscles. So, strengthen the abs and stretch the lower back. Due to her young age it should be much easier to address that now than later. And it will improve all aspect of her tennis performance.
Specifically for serving, once those muscles balance is improved, she will feel and understand how to push the hips forward which naturally causes the knees to bend just the right amount in the trophy position. Unless the hip (pelvis) is locked forward the coordination between the legs and torso is reduced for loss of power and control.