C.S.
Hi,
I believe you should take her to see an orthopaedics doctor who has a fellowship in pediatrics. You should also take in consideration that there are a number of people that just don't have the curvature in the neck. Being myself one of those people, I have been using for years one of those tempur pillows. You can just place a small rolled towel under her neck at night. A heating pad can help relax her neck muscles.
I would not take her to a chiropractor. There are some that believe that when a chiropractor make an adjustment it may stretch out ligaments without the body having the strength needed to support those changes. She probably will be better with physical therapy, where she will learn a combination of stretching and strengthening exercises. The ortho should suggest PT.
Good luck.