config.Routes.MapHttpRoute(
                           name: "OneLevelNested",
                           routeTemplate: "api/{parentController}/{parentId}/{controller}/{id}",
                           defaults: new { id = RouteParameter.Optional }
                       );

public class ChildController : ApiController
    {
// GET api/parent/1/child
        public IEnumerable<string> Get(ParentController parentController, int parentId)
        {
            return new string[] { "parent value1", "parent value2" };
        }
// GET api/values
public IEnumerable<string> Get()
        {
            return new string[] { "value1", "value2" };
        }
}