Acetylcarnitine, also spelled as acetyl-l carniitne, may help with attention deficit hyperactivity disorder (ADHD) in children with the genetic disorder known as fragile X syndrome. Fragile X syndrome results from an inherited genetic defect on the X chromosome. It is associated with  mental retardation and may also cause autism and ADHD. Italian researchers gave acetylcarnitine supplements to boys between 6 and 13 years old for a period one year and compared them to a group who took placebo pills. Those who took acetylcarnitine had a much better outcome. See and
I have not seen any studies regarding the use of acetyl l carnitine in children with ADHD who do not have fragile X syndrome, but I think the use of an acetyl l-carnitine supplement is a safer option to  prescription stimulants and is certainly worth a try. I am not sure what the ideal dosage of acetyl carnitine for long term treatment would be in children, but a dosage of 5 to 10 mg per kilo is a reasonable one to begin with This dosage can be adjusted higher or lower as needed. For a 30 kg child, (66 pounds), this would be about 200 to 300 mg per day. Inform your child's doctor about this study and both of you can agree on an appropriate dosage.