Children under age 4-5 often stick things up their noses, which can get stuck (a “thing” is called a “foreign body” in medical-speak). Then they have a bad-smelling chronic runny nose on just that side. The foreign body can usually be seen with a penlight, or an otoscope (instrument used to examine ears).
Removal can be tricky, because some of these “things” are hard to grab hold of with pincers. And the child is usually screaming and flailing during the attempt. Anesthesia may be needed.
But a very simple solution may work. A parent holds the child on their lap, pushes the opposite (clear) nostril closed from the side, and gives a gentle but strong puff into the mouth (like doing CPR). The breath travels up the back of the nose & back down, forcing the object out (see Diagram — Upper Respiratory Tract).