If a child has a fast processing brain eager to learn and crunch problems and a body full of energy, then when one or both of those has nowhere to go, they will act out.
Families have an abundance of mental stimulation cheap right on their phones, TV, etc. That likely makes both parents and kids less likely to move their bodies as much as the body required to generate natural endorphins and balance the needs of the brain. AKA: staring at screens or people all day and not moving as much as your body needs to will make you miserable, regardless of how successful your mental tasks were.
Carbs create more energy for the body and proteins with fat make more food for the brain, if one consumes more of one than they need and less of the other, they will also suffer. If you eat a bunch of carbs you will want to move more and think less.
A child or adult who gets to vary physical and mental exertion will be more focused and able to work. Test that. ADHD symptoms may not show as much in tired bodies telling you if the issue is that or an actual pathology in each person.