Gets the list of child rows for specified tree row.
|key (optional)||String||A value to determine the list type|
|row (optional)||Object||The row for which the list is requested|
An array that represents the list of child rows for the specified row. If row is not specified, a list of root rows is returned.
If key is set to 'selected', the list consists of selected rows only.
Supported in: v1.1.