Summarize:

If your child is under three years old, bow-legs will likely fix themselves.  Monitor your child as they grow and develop to make sure the bow in their legs is diminishing.  If you notice any irregularity in their gait as they begin walking, talk with your pediatrician.  Note that "watching and waiting" is the mainstay of treatment for young children with bow-legs. The key is to have regular check-ups with your child's doctor, to ensure that an intervention (such as casting the legs or, in severe cases, surgery) can be obtained promptly if they do not resolve on their own. Rickets disease, which is caused by a lack of vitamin D in a diet, is one thing that can cause bow-legs to develop. Increasing levels of vitamin D if they are low can help prevent Rickets from occurring and may help to correct bow-legs if already present.  Note that a vitamin D deficiency is not the cause of bow-legs unless your child has proven low levels of vitamin D upon testing. In other words, it may be the cause of bow-legs, but the two do not necessarily go hand-in-hand. It is advisable for your child to have their vitamin D levels tested to ensure that they are in the normal range, and to receive vitamin D supplements if they are not. Special leg braces, shoes, or casts may be used to treat bow-legs in young children, if they do not appear to be resolving spontaneously as the child grows. These are used if the condition is severe or the child has an additional disease in conjunction with bow-legs. The braces are worn by the child until the bones have been straightened.  Understand that this style of treatment is only used in severe cases. If needed, your doctor may refer you to an orthopedic surgeon for further treatment, such as surgery, for cases that cannot be corrected by the use of braces or casts alone. If you allow your child's bow-legs to persist through to adolescence, the picture can get much more complicated. The strain on your child's joints will be high due to the altered shape of their legs and knee joints. This can lead to pain in the ankles, hips, and/or knees. It can make it challenging to do prolonged physical activity, and it increases your child's chances of developing arthritis in later years due to wear and tear on his or her joints.
Wait and watch. Monitor vitamin D levels in your child's diet. Consider using medical braces. Understand the complications of failure to treat bow-legs.