Crossing the midline refers to the act of moving your arm or leg across the middle of your body to perform a task. It's a skill that is practiced and developed - like every other skill! - and it's critical for proper full development. Crossing the midline is vital to:

• The development of using both sides of the body together, such as putting on shoes and socks, writing and cutting;
• The promotion of coordination and communication of the left and right hemispheres of the brain.
• Encouraging bilateral coordination - the process of developing a dominant hand and development of fine-motor skills.

Before midline crossing is well established, children are usually observed engaging in tasks on only one side of their body, for example reaching for a block that is placed on their right side with only their right hand. They never use their left hand to cross that imaginary center line to grab an object on the right side. 

Kids who have difficulty crossing the midline can be seen:

• Switching hands during an activity like writing, cutting, or self-feeding;
• Rotating their whole torso (or trunk) instead of reaching across their body for objects that are placed on the opposite side; 
• Having difficulty performing age-appropriate self-care tasks independently (like tying shoes).

But, there are LOTS of simple, fun activities you can do with your toddler and little one to encourage practice with this important skill. And they don't have to feel like work, so encourage your kiddo to have some fun!

FOR YOUR TODDLER

• Bang objects together in the midline e.g.: hitting a tambourine.
• Throw, catch, and kick balls - encouraging them to practice with both hands equally.
• Put stickers on one side of the body and have your child remove them with their opposite hand (you can gently hold the hand you don't want them to use).
• "Wash" windows - give your little one a spray bottle of water or water + vinegar and let them wash the windows (or a table). Encourage them to make circular motions all over the area in front of them, with one hand at a time. (Side note: if you do this with your windows, be prepared to wash them again). 
• Place a cup above and to one side of their plate - on their non-dominant side; encourage them to practice reaching with their dominant hand. Then swap locations (and be prepared for spills).
• Give them spoons that encourage self-feeding then place the food on their plate or bowl such that they need to cross their midline to scoop it up.

FOR YOUR LITTLE

• Throw, catch, and kick balls! (This one works for all ages).
• Using a large piece of paper or chalk outside, draw a large figure 8 pattern and have your kiddo trace it with one hand.
• Create long tracks (with tape, blocks, chalk, etc) and have your little one drive her cars - one handed - across the "road". You can encourage them to switch hands and practice with both.
• Play games like Simon Says or Twister. 
• Simple activities like stringing beads, cutting, and folding paper also encourage crossing the midline practice.

There is a host of other ideas here and here and a handy printable with exercises here!