Nine times out of ten I grab my camera to catch a shot of my kids simply because of the the quality of the light I see. In this case it was because I noticed the Rembrandt effect from our kitchen window.
Rembrandt lighting was named after the master himself, who often used it in his own portraits.
The idea is to create a small inverted triangle of light on the subject that is opposite the light source. The triangle of light should fall near the eye on the upper cheek, on the less illuminated side of the face. It is important that you get catch lights in both eyes, without catch lights the image can appear lifeless.
This technique is typically produced with studio lighting, but can also be achieved by utilizing window light from one side at a 45 degree angle (as shown here). In this case I balanced the exposure between the highlights and the shadows (f 4.0, 1/80 ISO 400), I could have closed down my aperture or increased my shutter speed to create a more dramatic, shadowy effect, but for children I like to keep it a bit less serious!