Cannot sleep. My child has coarctation of the aorta. What is that?

A narrowing. The aorta is the main artery that comes out of the heart to deliver blood, oxygen and nutrients to the body. The artery has the shape of a "candy cane" and in some people the portion of the artery where the bend in the candy cane occurs may be narrowed. In some cases the narrowing will need to be relieved and in many cases this can be done with a cardiac catheterization and not surgery.
Stenosis. Coarctation of the aorta is a narrowing or stenosis of the proximal descending thoracic aorta and it commonly associated with other congenital heart disease -- so your child should be seen by pediatric cardiologist asap.