For Margaery: Wear a long, dirty blonde wig with your hair parted down the middle, or done up beautifully. If you're going for Margaery's time as a prisoner of the High Sparrow, just wear a plain, loose-fitting tan sack dress. If you're going for OG Marg, put on a beautiful blue gown as elegant as your updo.
For Tommen: See if you can find a robe or tunic in a gold/light-colored brocade fabric. Don't forget to wear a short blond wig, a golden crown, tall black boots, and a facial expression that suggests you hate your mother.