Bilateral coordination refers to using the two sides of your body together in a coordinated manner. This term can be used when both sides of the body are doing the same thing, such as rolling a ball with two hands, when the two sides are alternating to complete an activity, or when the two sides are doing something different, such as cutting a piece of paper.
Bilateral coordination is a developmental skill that appears in early childhood. Babies tend to use symmetrical movement early on, moving both hands together or both legs together. As the child begins to develop, reciprocal movements begin to emerge, such as the alternating of arms and legs in crawling. Reciprocal movement is required for the child to walk, run, skip, or ride a bike. The next skill that emerges is asymmetrical bilateral coordination. This skill is seen when a child cuts paper, holding the paper with one hand and maneuvering the scissors with the other hand. It's also needed for coloring, writing, kicking. In each case, one arm or leg is active while the other stabilizes. Children tend to have some proficiency at this skill by age 4. The next skill to develop is crossing midline. Crossing midline refers to when you use your right arm or leg on the left side or left arm or leg on the right side. Basically it's any time you cross the midline of your body with an arm or leg. Prior to the development of this skill, children tend to use their right hand when things are on the right side and their left hand when things are on their left side. In other words, they have not yet developed handedness, or hand dominance.
By age 3-4, children generally have a strong hand preference, but they continue to switch off. By age 4-6, hand dominance is typically established and should be apparent and consistent.
Though you want a strong hand dominance to develop, it's also important that the child use their other hand to assist. Encourage activities that require them to use both hands as well as those that help to develop hand dominance.