I'm a teacher. I've spent decades working with ADD/ADHD students.
I'm also a daughter of a pretty extreme adhd woman, and the grandmother of an adhd boy.
Back in the 90s, I was teaching 8-9 year olds. My duties included PE. I taught in an area that, for months, was too hot to take kids outdoors...by law. So I developed all kinds of indoor activites instead. For PE, we would dance, do yoga, or play catch while sitting on tables, or do other physical things, but it wasn't enough. EVERYBODY needed frequent breaks and help with self-management when we had to spend all day indoors. Some of the dance and yoga that we did could be utilized for quick 2-minute breaks. I also did meditation breaks.
I didn't call them that. I used a book: The Boy and the Bear by Lori Lite. A "children's relaxation book." I would have them stretch out on the carpet and read to them; they would follow the instructions/actions that the boy was doing with the bear in terms of breathing. It was basically an exercise to teach meditation: breathing and emptying the mind, to young people.
It helped that, at that time, we had a 20-1 student-teacher ratio, so the room wasn't too crowded. Still, I took careful note of the results. Some kids immediately fell asleep. Some kids quickly became masters at relaxation exercises. Some struggled but got there.
The adhd kids? It was a torment to them. The very idea of having to be still, quiet, and to control their breathing had them twitching and squirming. They never mastered it, and always breathed with relief when we were done.
When the weather cooled down, I tried again. I took them out on the field and had them spread out on the grass. I made sure the adhd kids were on the edges, with more empty space between them and other people than we could manage inside.
It worked, and it worked quickly. And they were relieved. So was I. I asked them if they thought they could use relaxation and breathing to help them focus when they were struggling in class; it didn't have to be lying down or while I was reading to them. They wanted to try, and reported that it did help some.