This image was shot with a Nikon D40 and a 1.8 50mm lens at a focal length of 35mm.
Shutter Speed 1/125
This image looks a little under exposed to me. After you take a photo look in the LCD display on the back of your camera and see how the image looks to you. The most important part of this image, in terms of correct exposure, is the boys face. So when you look at the display, look at how well exposed his face is. In this case his face is too dark so you need to let more light into the camera. In your case your shutter is about as slow as you would want it when a child is your subject so you can either increase your ISO or open up your aperture (to a lower number). When opening up the aperture you’ll have to be more careful about making sure you focus on his face (assuming that is what you want in focus). If you accidentally focus on his hand or the airplane then his face could be blurred. Just something to be aware of.
The lighting here is great. It is nice soft light and you have him nice and close to it. If you happen to want him to turn more towards you then you might want to find something to use on the opposite side of him (opposite the window) to bounce the window light into the darker side of his face. You can have someone hold up a white sheet or towel or anything white or light in color.
When composing an image I always think of the photograph as a story. When telling a story you have to think about what to include and what to leave out. What is the story of this image? Boy looking out the window with his toy? If the toy is part of the story then perhaps consider including more of it in your shot. Perhaps a little more of the right arm and hand could be shown. You could consider moving your angle down a little lower or even a little lower than his face and take the shot that way.