What feature of the skeleton eventually allow a child to maintain balance while walking and running?

The vertebral column, when seen from the side, has 4 curves: 2 convex curves (cervical and lumbar curves) and 2 concave (thoracic and sacral curves). This curves are the feature of the skeleton that allow a child to maintain balance in the upright position eventually.

When we are born we have a single concave curve throughout the whole vertebral column but, as we grow, still in the early months as we try to hold our head, the cervical curve starts to develop, and later, as we try to sit up, stand up, and walk, the lumbar curve aslo starts to develop. In few years we have all curves fully developed.