As this paper is from 1997, I'm posting more recent support for it ...
Magnesium supplementation in children with attention deficit hyperactivity disorder - published 2016
Magnesium Status in Children with Attention-Deficit/Hyperactivity Disorder and/or Autism Spectrum Disorder - published 2020
Magnesium status and attention deficit hyperactivity disorder (ADHD): A meta-analysis - published 2019
Magnesium status and attention deficit hyperactivity disorder (ADHD): A meta-analysis - published 2019