You can request a GAC (Guest Assistance Card) at Guest Services at any of the parks. They are not meant to (and usually won't) actually shorten or eliminate your wait in line. What they are meant to do is provide the assistance that someone with an invisible disability needs to enjoy the park as they would without a disability.
You can go to Guest Services in any of the parks (the person with a disability needs to be with you). Each situation is looked at on a case by case basis - they don't just give the card based on a diagnosis. Because each person's needs are different, some people with adhd won't need a GAC at all, some might need one only to use on one or two occassions a day and other people might need the GAC more. Be prepared to explain what sorts of assistance you need -like do you need a quieter place to wait, use a stroller in line to help cut down on stimulation, etc. A GAC is usually issued for your whole length of stay (same GAC valid in all parks) amd is for up to 6 people (5 plus the person with a disability). There are certain places where they will limit the number of people (some rides have small waiting areas and only 2 or 3 will fit.
We use a GAC more like insurance and even at Spring break time, we use it sparingly and try to avoid using it by looking for short lines and using fastpass.
Fastpass will be very helpful to you. It is available to anyone at no charge. You put your park ticket into the fastpass machine and a fastpass card pops out with a return time. When you return, you will usually have less than a 10 minute wait. If you can get to the park early in the morning, you can also get on a lot of things with a very short wait.