How can I focus quickly on kids running outdoors at night?

Your camera needs more light to focus. There are a few ways to add light in your situation:

• Use a lens with a wider maximum aperture. Remember that metering and AF are done with the lens wide open. It isn't stopped down to the set aperture until the instant before the shutter opens. A fast 50mm f/1.8 lens is the probably the most budget conscious choice.

• Use a flash (or wireless flash transmitter) that has an IR focus assist lamp. Although their effective range is somewhat limited, they can be effective when used at typical casual portrait distances.

• Use a directional external light source such as a flashlight/hand torch to illuminate your subjects for focusing. With small children an assistant that can follow them with the beam of the light might come in very handy!

Additionally, most cameras' AF systems have center focus points that are more sensitive and more accurate with wide aperture lenses in low light. Manually selecting the center focus point may help increase the speed at which your existing lens can focus in marginal conditions.