A There's probably no need: Most kids have pancake-bottom feet until they're about 3 years old; after that, an arch should begin to develop. Once your child is that age, you can check his arch by watching him walk on concrete with wet feet; the concrete should be dry in the area of the arch. If it isn't, he may need an evaluation. How to tell: Have him stand barefoot on a hard surface. Stand behind him and place a ruler along his Achilles tendon, which runs from the back of the calf and ankle to the heel. If the line of the ruler is perpendicular to the floor, the flat feet seldom need treatment and generally don't bother the child. But if the line angles inward, with the heel turning outward (called pronation), it can be a cause for concern and you may want to see your pediatrician.
Some orthopedists take a more conservative "leave it alone, he'll outgrow it" stance. But in my practice I've seen many children and teens who've developed sore knees and ankles because their flat feet have thrown off weight distribution and, consequently, put too much pressure on the joints. (This can be particularly troublesome for obese kids, since the extra weight adds even more pressure.)
Treatment is simple and can begin as early as age 3: A soft orthotic (an arch support available from a podiatrist or at some children's shoe stores) slips comfortably into a child's shoes. He should wear them every time he has shoes on, for a minimum of four to six hours a day.