This advice ought to be basic knowledge, but every day I notice portraits that are out of focus. When you look at a portrait, the first thing you notice is the eyes. If the eyes are out of focus, the entire photo look out of focus. If the child don’t look straight at you with both eyes in the same distance from the camera, you need to focus on the closest eye. And with focus on the eye I don’t mean something close to the eye. It’s not the eyelashes, not the nose or the cheeks that should be in focus. You need to focus spot on in the center of the eye. When making portraits you often use a very narrow depth of field and long focal lengths. Thus you require to be very precise in your focusing. Especially if you move your camera to change the composition after your focusing. Check the results on the camera to make sure that your focus is right. You can’t fix an out of focus eye in your postprocessing and it will ruin the photo completely.
Crisp and clear eyes are my primary concern when I make portraits. If the details in the eye don’t make me loose my breath I will usually discard the photo. Of course – there’s always exceptions, and you can of course use out of focus portraits for creating certain kinds of moods and expressions, and it can sometimes be more important to catch the right moment than just the right focus. But as a rule of thumb this is /the/ most important part of the portrait. Practice, practice, practice