@HTML.Partial("~/Parent/Child1/1.cshtml")

@HTML.Partial("~/Parent/Default/1.cshtml")

var razorEngine = ViewEngines.Engines.OfType<RazorViewEngine>().First();
        razorEngine.ViewLocationFormats = razorEngine.ViewLocationFormats.Concat(new string[]
        {
            "~/Parent/Default/{0}.cshtml"
        }).ToArray();
        razorEngine.PartialViewLocationFormats = razorEngine.PartialViewLocationFormats.Concat(new string[]
        {
            "~/Parent/Default/{0}.cshtml"
        }).ToArray();