How can add search on second row or child row?

The rule that determines which rows will be displayed when a row filter is applied is implemented in onRowFilter event handler. You'll need to extend it to include text of the additional rows as well.

Please note that there is no built-in onRowFilter implementation. The reason is that everyone wants a slightly different behavior.

See also:
https://doc.daypilot.org/scheduler/row-filtering/