But as an experiment, because of this thread, I tried it. When using the viewfinder with AF-A and Auto, facial recognition worked maybe once in 5 or 6 shots. Otherwise the focus point was all over the place and in multiple places.
It was better in LiveView as when you acquired facial recognition focus the focus square would show on the screen and on the face. It's pretty slow though and not designed to chase little kids around. This is the first time I used face recognition and while I find it's easy to set up it's not very reliable. Diane has the right idea.