Use the optical viewfinder instead of the LCD monitor when you can. Added to the already slow shutter speed, this means that something might have already happened by the time you see it on the LCD monitor. Use spot metering instead of matrix for most subjects. This will not work for close range shots of running kids, but you can try it. If you can, use a flash.
Nothing freezes the action like a flash.
An external flash is best. Shoot at a resolution lower than Fine. That is because the shots will not be perfectly sharp due to motion anyway, and you want to get the shortest shot to shot time and the most buffered shots. Just keep the best ones. This is a very unusual 990 tidbit, and I'm not sure where to put it, so I'm putting it here on the tips page.