Preferential seating is often offered, but in very specific sense. Usually it meas front row center. This is great in some respects, but other options are better for other types of kids. For example, those of us who benefit from movement, might be better off on the side of the room with the understanding that it would help us to stand up at certain time during the class. I always liked to be up front but by the side so I could go for a walk without feeling like I was disrupting class. When I take classes, go to conferences, check out lectures at this point in my life, I prefer to grab a seat in the back and end up either standing up for much of the session or even sitting on the ground in the back to afford me the most possibility of moving around.
My point is that most kids could benefit from targeted seating, but that might not be the same for every kid with ADHD. It may even vary from class to class. And, it may be somewhat depending on who else in in the class and how long it is. Think it through with your kids. They might have an insight into this that you wouldn’t have thought they’d have.