When I work with children, I often try to make as much eye contact as possible instead of burying my head behind the viewfinder. This works well with adults, but is particularly important with younger ones.

If a child is timid or shy, including them in the process—letting them see a few shots, allowing them to take a picture—can warm them up to you, but be careful, this can backfire if,  after that, all they want to do is play with your camera.

Keep in mind that their attention span is short which limits the time you can spend taking controlled portraits. When they lose interest in sitting for a portrait, follow them around for awhile, capture them in more candid settings. These are often the best shots.