Alternative (supplements or vitamins) to meds

Let me preface this by saying I am NOT against medication but not allowed to medicate the child that brought me to this thread seeking for help.

My 12 yr old niece has ADHD, parents REFUSE medication but also, do not help her with homework, routines, ANYTHING. They are completely neglectful parents and I do spent a lot of time with the child, helping her out with school as well as taking her to swim lessons and overall hanging and bonding with her. We are pretty close but i am not her primary caregiver and I will not medicate a child that is not mine. I have spoken to her parents and they are willing to try supplements or vitamins; as long as its over the counter, they are good with it.

So i will take any suggestions. Given her age, and weight, im not sure if a "child" supplement is actually needed, as she could take adult doses.