I am a long-term migraine sufferer and can empathize with my patients that have migraines. I have been on a number of treatment and preventative medications and know that it can be difficult to deal with migraines. It has taken me a long time to figure out my triggers (journaling about my headaches helped).
I am open minded about recommending different preventative measures if they have been scientifically tested. One herbal supplement that has been scientifically tested is butterbur. A purified form without PAs (pyrrolizidine alkaloids) and 8mg total petasin called Petadolex(r) is the one I would recommend over alternatives. Side effects are considered minimal and include belching, itchy eyes, diarrhea, and headache(!). Butterbur should not be taken by those with ragweed allergy or a history of liver problems.
For migraine prevention, the starting dose is 50mg capsule twice a day. The dose can be increased to 75mg twice a day if needed. One average, there were less headaches and less severe headaches, a decrease by about half. It has studied in children down to 6 years of age with success.
If your child has migraine headaches, discuss with your pediatrician whether this may be something to try.