The commonest cause of U/L mucopurulent rhinorrhoea in a child is -

"A unilateral nasal discharge is nearly always due to a foreign body and if the discharge has an unpleasant smell, it is pathognomonic". — Logan Turner 10th/e p. 63 "If a child presents with unilateral, foul smelling nasal discharge, the foreign body must be excluded". — Dhingra 5th/e p. 176