A princess face isn't something you can mess up very easily, because unlike a panda or a cat or a teddy bear, a princess face isn't something specific you need to mimic. Anything pretty and in a color the girl likes should go over well.
Princess Palette
Pick your colors. Most girls will prefer blue, pink or purple. If there is no preference, match the color to the clothes being worn. Try not to mix blue, pink and purple together--limit your palette to shades and tints of one color only, with highlights of white. Flourishes of gold or silver may also nice, in small amounts.
Princess Base
Consider the structure of the child's face before proceeding. An emphasis on the eyes and cheeks will complement most little girl's faces nicely. Since a princess does not actually look any different from any other person, the princess face will mostly be a concoction of made up swirls, curls and feminine designs like stars and hearts. Make your design symmetrical and radiating outward from the center. Give the girl eye shadow and line the eyes with white. Little curly cues emerging from the outer corners of the eyes will give the child a cute and somewhat mystical appearance.
Princess Crown
Paint a jewel or a star in the center of the forehead. If the child will not be wearing a crown, you may wish to make the jewel connected to a line that proceeds into the child's temples, to imitate the presence of a crown.
Princess Finishing Touches
Have the child look at herself in the mirror and ask her if she has any suggestions. For the finishing touches, give her a little paint on the lips (a lighter color is usually better than a darker color) and top everything off with glitter dabbed on the face.
