What lens are you using? Zoom or a single focal length?

I'd probably shoot a wide angle stopped down to f/5.6 and leth the depth of field of the lens take care of any minute focusing errors.

If you're shooting a 50mm lens at 3 feet, for example, depth of field at f/2.8 is only three inches. A 35mm lens at the same aperture and distance will have six inches depth of field.

At 2 feet, a 50mm lens will have just one inche depth of field, while a 35mm lens will give 3.3 inches.

If you stop down to f/4 or f/5.6, you have more room for focusing errors, which will happen when you're trying to follow an active child.

You'll find that shooting wide open or close to it is best for static objects, people who don't move too quickly or when you can get someone (child or adult) to stop moving for a few moments.