When deciding on curtains for children's bedrooms, take account both the purpose they will serve and the décor of the room. If your child's room is very bright in the mornings, you might want panel drapes with blackout lining to keep the room dark for longer. If you already have effective shades or blinds, you might just want a decorative top-treatment.
If you have a small room, you might want a more minimalist window treatment, such as a Roman shade, so that the room is not overwhelmed by fabric. Look through some magazines and catalogs to get a sense of what will work for your child's room, and adapt it to suit the décor.