The video below in Thai language shows how phlegm suction is performed on a child. In the description of the video, it is mentioned that the video may be hard to watch and it may seemed the action hurt the child a lot as she was crying. However, you can see that the child had A LOT of mucus and phlegm.
If it is not being suctioned out, the child would have difficulty breathing. The nurse did nasal as well as throat suction. You can hear the sounds of phlegm during the suction. The child had very thick mucus in her nose which the nurse had to use saline water to wet before she was able to do suction.