I generally don't give advice on these boards about healthcare, but I guess I'm about to make an exception.
I'm a chiropractor, strictly musculoskeletal. I look at what I do as more or less rehabilitation. I have treatment plans, and release patients following those plans. Generally a course of treatment is no longer than 2 weeks (or 6 treatments).
I've never seen any research that correlates ADHD to the cervical spine. That being said, stranger things have happened. Generally, if you think something is going to work, it probably will. It happens with placebos all the time. I'm not calling chiropractic a placebo. I hope you get the drift.
Are there side effects to cervical spine manipulation? Of course there are. They are rare, but they are nonetheless there. Informed consent is the key. If you are told of the possible side effects, as well as other treatments options, then I think you are the best to decide what form of treatment to follow.
Again, I practice strictly musculoskeletal. I'm not a neurologist, psychiatrist, etc. Personally, I would never tell someone I could help their ADHD. I've also never adjusted my children.
I know some people think all chiropractors are quacks. I just want to tell you that more of the younger generation D.C.s are practicing evidence based healthcare.
Good luck to you.