Configure the advanced settings of the Face Detection module as follows:

1. Go to the Additional settings tab (1) on the settings panel of the Face Detection object.
   
2. Set the Pixelize area checkbox to pixelate the part of video image in which the face was recognized (2).

3. Set the Grain size cursor in position corresponding to the pixelation grain size in pixels (3).

4. To save the changes, click the Apply button (4).