D7000: Best Focus Mode for Photos of Toddlers?

I have a D7000 and want to photograph our grandchildren who are toddlers. These kids are in perpetual motion.  I was wondering if anyone has found a good focus mode or method for doing this.  Would 3-D tracking work best or one of the AF-C dynamic focus area modes. If the latter which one would work best, 9, 21, or 39?   I would like to take advantage of the high ISO capability of the camera and use available light if possible, but would resort to my SB600 if necessary.