There are 2 things you should think about.
1. She might do best being able to stay in her stroller even in waiting areas. At Guest Relations, they can mark your Guest Assistance Card (GAC) or give you a hang tag for the stroller indicating that you can use your stroller as a wheelchair meaning that you can bring the stroller anywhere that a wheelchair can go. The stroller will allow her a safe place to retreat from the noise and lights and motion of the people, especially if you have some kind of canopy. In addition, it can keep her from suddenly bolting from you or from going down on the ground and refusing to move. I strongly recommend this.
2. Because many lines are wheelchair accessible, this will not be the only accomodation that you need. Explain to the CM at Guest Relations exactly what you explained here about waiting and sensory needs. I would include the fact that even at school she doesn't wait in the hall with the other children. The "standard" accomodation for kids with sensory needs who have trouble waiting still do have some wait time and typically can handle this. I'm not sure if there might be another level of accomodation that would be possible for faster access to rides since it sounds as though she has a much shorter meltdown point than most with these kinds of difficulties.
As the other person said, you do not need any kind of doctor's letter. In fact, the person at Guest Relations really doesn't want to see it since a diagnosis doesn't tell him anything about the actual need plus they are not allowed to ask for one I believe due to HIIPA laws. You need to be able to explain to the person at Guest Relations exactly what kinds of difficulties she has and he will give you a GAC which provides for her needs. GACs are for people who have invisible types of disabilities. Cast Members all over the parks are used to seeing them and in no way question the validity of the need based on appearances. They're trained to understand that GACs are given to people with invisible disabilities.
If your son has his own special needs, get a separate card for him. For rides that your daughter won't ride, you can't use her GAC. It's only applicable for the person who is on the card and up to 5 guests. If he doesn't have any particular special needs then ignore this advice. I can't tell if he does or doesn't based on his diagnosis because each child with ADHD will have his/her own unique needs (again, this is why the doctor letter stating a diagnosis is useless for the people at Guest Relations).
Even with the GAC and stroller as wheelchair option, don't plan to do commando type tours of the parks. Try to go when it is slowest (rope drop at the parks and Extra Magic Hours are great) so there won't be so many people around and waits are going to be shorter. Try to eat at off times so there won't be so many people in the restaurants. Watch her closely for early warning signs of meltdown because it does take a while to get back to your hotel and you want to be able to head out long before full meltdowns happen.
