Well, you could go a couple ways with this. You don't want the cheap brushes that come with children's painting sets. In the painting section of most art stores there's a selection of brushes in all sizes. Don't buy the cheapest, it will lose bristles; pick a middle-priced brush, care for it well, and it will last a long time.
But if your hand isn't the steadiest or you have many identical faces to paint, you might experiment with creating a polymer clay stamp for different features, and stamping them on the face. I'm not sure how well the usual stamp pads work on clay; if they're water-based they'll just bead up. Try acrylic paint instead. Work fast; it dries quickly. If you keep a wet paper towel handy you can quickly wipe off any mistakes.
Hope this helped!
