For Margaery: Wear a long dirty-blonde wig with your hair parted in the centre 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-coloured brocade fabric. Don't forget to wear a short blonde wig, a golden crown, tall black boots, and a facial expression that suggests you hate your mother.