How to handle hierarchical routes in ASP.NET Web API?

Configure the routes as below. The {param} is optional (use if you need):

routes.MapHttpRoute(
           name: "childapi",
           routeTemplate: "api/Parent/{id}/Child/{param}",
           defaults: new { controller = "Child", param = RouteParameter.Optional }
  );

routes.MapHttpRoute(
         name: "DefaultApi",
         routeTemplate: "api/{controller}/{id}",
         defaults: new { id = RouteParameter.Optional }
  );

Then call the child APi as /api/Parent/1/child
The parent can be called simple as /api/Parent/

The child controller:

    public class ChildController : ApiController
    {     
        public string Get(int id)
        {
          //the id is id between parent/{id}/child  
          return "value";
        }
        .......
    }