What are the consequences of coarctation of the aorta in a child?

High pressure. As with most things it depends on the severity and age. Coarctation is a degree of narrowing or obstruction in the arch of the aorta. As a result the part of the aorta before the obstruction usually has higher pressure than after the obstruction. So the heart and the arteries going to the head and the r arm usually have high pressure and may cause sometimes severe and irreversible complications.
Hypertension. Untreated coarctation causes severe hypertension. Left untreated, severe hypertension can lead to stroke, aortic aneurysm, aortic dissection, and congestive heart failure. Luckily, in the modern era, coarctation is manageable.