If you’d be willing to go to Bellevue, Gayle Fay PhD is good. She provides a plethora of services, and I believe one of them is diagnosing kids as well as adults with ADD/ADHD. She runs NW Neuropsychology, Learning & Behavior Services. Her contact info is:
Gayle Fay, PhD
Northwest Neuropsychology
2227 112th Ave NE Ste 202
Bellevue, WA 98004-2953
Phone:
(425) 452-8036
If nothing else, they may be able to give you a referral for closer to Seattle. I hope that helps!