Talk to your child about your plans to apply whiteboard paint somewhere in the bedroom and discuss the best places.
Ask your child to stand in the location where the whiteboard paint will be applied. Make pencil notations of your child's reach to determine the locations for the bottom and the top of the whiteboard surface.
Mask off a large square over the area where the white board paint will be applied with painting tape. Don't feel limited to painting a square or rectangle on your child's walls -- ask your child what shape to use. The top of the whiteboard area should be at least 3 or 4 feet above your child's reach to ensure that she will have growing room, so that when she is several years older she can still use the white board paint as a drawing surface.
Paint the area and give it time to dry. Remove the painting tape from the edges and touch up any areas as necessary.