So how can you capture those pictures you find when you buy a picture frame? I don't know about you, but my nephews are constantly moving, running around, jumping, and if you aren't in the right place, at the right time, with that split-second reaction, you won't capture the picture. So how do you do it?
There are really two elements to it. The first is lighting. Kids move around so much, and I know in my house, while the main two rooms are lit very well, my nephews wander around so much, and one of them in particular, prefers the darker rooms. So you need to make sure you have good lighting. I personally like my SB-600 for this. I usually leave it in portrait orientation, (one of the advantages it has over the SB-400) pointed at the ceiling. This allows for the more natural lighting look, and if you are photographing after dark, it can completely black out the background, while making it look well-lit. So while it might seem pointless, even in a well-lit room, you should still use the flash. The eye has a way of deceiving the brain, and making it look like there is more light than there really is.
The second aspect, and while it may seem slightly more of a no-brainer, is to not worry about the equipment too much. Don't bother switching lenses to get the optimal shot, putting filters on, etc. This will only cause you miss the one shot you need to get. Also, you will want to use a zoom lens. Unless you are willing to keep trying to get that one shot out of a hundred, use a slower zoom lens so you can keep up with their movements. And this part really depends on what type of photographer you are. I prefer to stand back, and not be at the center of everything, snapping pictures. As such, this means I use my 55-200mm VR primarily, because it has such a wide range, but it also can be used in darker situations.
You also will want to make sure you have batteries. Most of the time, you will most likely be waiting. This means you will be watching the kid, usually through the camera, constantly repositioning yourself and readjustin g your camera settings, primarily the zoom, to match the surroundings. This wears your battery down, so you will want to watch the level as the day or event goes on.
camera will constantly be readjusting the focus as the subject comes closer or further. You can change this by pushing the closest button the top of your D90, that has the letters 'AF' printed on it. You'll have to hold that button, while spinning the back scroll-wheel. (Note: Most people usually already have the camera set to AF-A; The reason for switching to AF-C is because kids can stand still, your D90 isn't as smart as you, and might assume they are standing still. Trust me, I have had blurry pictures of kids come out because they started moving, doing something worthwhile capturing, and the camera refused to change fast enough.)
1) Lighting: Usually your own, but watch for the ambient light.
2) Lenses: Use a zoom lens. This way you can capture within a wider area.