Many types of children’s flat feet don’t cause painful problems. When the low arches are the result of a condition like tarsal coalition, however, it can be quite uncomfortable for your child. Tarsal coalition is a bone abnormality in the middle or back of the foot. Extra bone, cartilage, or other fibrous tissue grows between two of the tarsal bones, connecting them. While your child is young and growing, this connection is usually relatively flexible and painless. As your son or daughter gets older, the bones harden more and the feet mature. This extra tissue hardens as well, making the arch rigid.