choose the theme based on likings (My daughter is 6 and love the living things right now she likes cute cartoon animals, butterflies, birds, snails, flowers etc...
and then apply it on the focal point of the room (Focal point is the wall or place which you see the first when you enter the room) and colour your room which can go with your theme.
you can choose to do accent wall (the wall which cant be seen until you go fully in the room, it will add magic to the room and that could be any bright colour which can go or popout with your choice of room colour.
