Minimalism is definitely the word of the day for her. Unless she ABSOLUTELY needs it, you probably shouldn't do a full face, unless her character is the sort of girl that's really into makeup. Youth makeup and men's grooming are in very much the same vein, to my mind. Just fix the obvious stuff, don't get too jiggy with it.
If she IS a girly girl that's into getting made up, then still keep a very light hand on the skin, and focus on eyes and lips, maybe a liiiittle bit of contouring. The contouring is what usually matures someone of that age. It takes away that youthful roundness.