I found a problem,UltraWebGrid's child table can't achieve by user-defined column. Why?  When I set AutoGenerateColumns is true, UltraWebGrid's child table can display.  Below is my C# code:

protected void uwgLight_DataBinding(object sender, EventArgs e)
    {
        DataSet ds = Class_LightShelf.GetAll(0);
        ds.Tables[0].TableName = "LightShelf";
        ds.Tables[1].TableName = "LightSeat";

        ds.Relations.Add("LightRelations", ds.Tables["LightShelf"].Columns["ID"], ds.Tables["LightSeat"].Columns["LSID"]);

        this.uwgLight.DataSource = ds;
    }

protected void Page_Load(object sender, EventArgs e)
    {
        if(!IsPostBack)
        {
            this.uwtLight.DataBind();
        }
    }