There are several types of flatfeet. The most common is the familial flatfoot; it is flexible and generally is not a source of pain. In the past, such painless feet were treated with special shoes, wedges, and cookies, all in an attempt to correct the flatfoot. This type of treatment is no longer recommended, because is does not work and is not necessary. The average height of the arch is lower in the child than in the adult. The height of the longitudinal arch increases spontaneously during the first decade of life in most children, and there is a wide range of normal arch heights at all ages, particularly in young children. The arch in a child’s foot matures over the first 12 years of life. The diagnosis of flatfoot cannot be made until a child is at least 2 years of age. Ligamentous laxity (flexibility) is the primary thought behind the flattening of the arch in a child. Some children with flexible flatfeet have activity-related pain in the leg or foot that is relieved by shoe inserts. If your child’s foot is painful, your doctor will evaluate the foot to determine the cause of the pain. There are several types of foot problems that can be painful and that may be associated with the non-flexible flatfoot.

More information:
http://kidshealth.org/parent/medical/bones/common_ortho_p2.html