Answer to Question #92114 in Other English for Thato
If you were to draw a line down the middle of your body, starting at the head, that is your midline. Every time you cross that line with either side of your body, that is crossing midline. Crossing midline is a skill that children can learn from infancy.
Poor mid-line crossing will affect how your child reads (tracking with the eye from left to right) and writes (using their dominant hand across the writing page).