Child Table,different column options for different rows in custom script

1 
      var childTable =frm.add_child("parameters_table"); 
     childTable.idx=1
    childTable.column1="a__" 
    childTable.column2="b__" 
frm.refresh_fields("parameters_table")

    var childTable =frm.add_child("parameters_table"); 
    childTable.idx=2
    childTable.column1="d__" 
    childTable.column2="b" 
frm.refresh_fields("parameters_table")

How to set options for second row (idx =2) ,column2 like [“b”, “c”, “d”] , so user will select one of them