Once the gap between your child's knees closes up, she may develop the opposite problem around age 2. Kids' knees can start getting closer together for a variety of reasons -- sometimes the bones overcorrect a little after they've stopped bowing, explains Sara Rizvi, MD, a pediatrician at Texas Children's Hospital, in Houston. Although knock-knees look painful, they usually aren't. In the vast majority of cases there is no need for concern, and your child will be able to run and play normally. Her legs should line up better by the time she's 5 to 7.