In context of child row processing, row.child is undefined.

I am trying to add a child row to my dataDatble. Toward that end I am using the standard code block you show provide to show and hide child rows:

    $('#example tbody').on( 'click', 'tr td.details-control', function () {
        var tr = $(this).closest('tr');
        var row = dt.row( tr );
        var idx = $.inArray( tr.attr('id'), detailRows );

        if ( row.child.isShown() ) {   // ============ here row,child is undefined !!! 
                                                   // The row node does not include a child element 'child' ...==============
            tr.removeClass( 'details' );
            row.child.hide();

            // Remove from the 'open' array
            detailRows.splice( idx, 1 );
        }
        else {
            tr.addClass( 'details' );
            row.child( format( row.data() ) ).show();

            // Add to the 'open' array
            if ( idx === -1 ) {
                detailRows.push( tr.attr('id') );
            }
        }
    } );

The code fails because the var row does not contain a child element 'child'. What am I missing to enable child rows in my DataTables?