Just imagine a line down the centre of the body. Crossing the midline is a child's ability to reach across the middle of the body with arms and legs crossing over the opposite side. It's an important developmental skill needed for writing, putting on shoes and socks, hitting a ball with a bat and more.
The following article Crossing The Midline provides information on Why Crossing The Midline Is Important, Signs Of A Child Who Has Problems Crossing The Midline and Activites to Improve A Child's Ability In Crossing A Midline and more.
Read the article here: Crossing The Midline