First of all, keep in mind that I am unable to diagnose you because I am unable to examine you, this forum is for educational purposes.
Your daughters case is very complex and needs an expert that can take a more extensive history and examine her. I agree that she will need an MRI of her cervical spine to help find the reason for her pain, but there are several different possible etiologies for the pain, each of which would require different treatment. One thing you should look out for is stiffening of the legs, weakness in the arms/legs, and loss of bowel and bladder control (which might indicate that there is compression of the spinal cord). There is more things that you can do for pain, but this should be conducted through your neurologist or pain specalist.
I hope this has been helpful.
[First of all, let me state that I am not a doctor, just a neuropathic pain sufferer for 28+ years, diagnosed with a possible variant of NF1 about 14 years ago. I have learned to become my own health advocate, and am very active in decisions concerning my treatment plans.]
I am sorry to hear of your daughter's pain. As you probably know, the plexiform neuromas and other lesions can grow almost anywhere along the nervous system, even though the most obvious lesions are those which we can see.
The vertebrae in the neck have small openings through which the passage of sensory nerves can get quite crowded (particularly in the distribution of the C2 sensory nerves). Any neuromas growing in this area can compress the affected nerve or other nearby nerves, resulting in considerable pain. Ibuprofen is probably the best over-the-counter treatment as long as it is not overused. Otherwise, try heat or cold compresses, and give your daughter a comfortable place to lie down. At her age, she can start to learn techniques such as progressive relaxation and bio-feedback.
Symptoms to watch out for are progressively increasing pain that does not respond to treatment or is accompanied by excessive neck stiffness and restricted range of motion.
Follow your pediatrician's advice, and continue to research this disease as much as you can (it sounds like you're doing a pretty good job already!)