When I updated the children records in relatated one-to-many tables and call $parent->save(true) I want that parent record was updated too. But it was not changed and not updated. Can I force updating of this record in any way?
The createdat observer will only be called on an INSERT, never on an update.
The updatedat observer is called on every UPDATE, and will also update the updated_at column is a related and loaded child record has changed. This only works if the client object itself isn't saved first, as it will call is_changed() on all related objects, and that returns false after a save().