Large adenoid and tonsils are common cause of snoring in children.This can be associated with a condition called obstructive sleep apnea-stoppage of breathing while asleep.
X-ray, CT imaging and nasal endoscopy are the the investigations commonly done.Only older children tolerate nasal endoscopy without anesthesia.
(This video demonstrate how the nasal endoscopy being performed in a child in Jubilee Hospital, Trivandrum, India)