AF setting for kid portrait
I am planning to take photos for a kid in a studio setup and wondering if I can get some suggestions about AF settings. My camera will be on a tripod, kid will be on white seamless background. I will use radio triggers to trig both camera's shutter and speedlight. Since I won't be looking through the viewfinder, what I want is the camera to focus on the nearest subject (the kid) when I press the trigger. What AF area mode is most appropriate for this? I am using D7000 and my camera has 1)single-point AF; 2)9-pt dynamic area; 3) 21-pt dynamic area; 4) 39-pt dynamic area; 5) 3D tracking; 6) Auto-area AF.
I am intending to use Auto-area AF with AF-S.