#16. "RE: HELP: First day with D800 and already in panic!" In response to In response to 15
VR is activated with the AF-On button and will continue for a few seconds. It activates pretty quickly. You have two options - continue to hold down the AF-On button and VR will continue and AF will track the subject, or focus and activate the shutter with activity and take your chances. I probably would simply hold down the AF-On button and shoot bursts of several frames if necessary.
You will have no problem using a higher ISO if necessary. Noise at higher ISO levels is very low. VR will help with camera motion, but not subject motion. 1/160 sec should be fine but if your son is very active, a faster shutter may be required.