The commonest cause of intestinal obstruction in children is –
Intussusception 

Intussusception occurs when a portion of the alimentary tract is telescoped into an adjacent segment.
It is the most common cause of intestinal obstruction between 3 months and 6 years of age.
It is more common in males.
Intusssception may be →

i) Ileocolic (most common)    
ii) Cecocolic                         
iii) Ileoileal

The upper portion of bowel that invaginates into lower portion is called intussusceptum.
The lower portion which receives telescoped upper portion is called intussuscipiens.
Intussusception
Volvulus
Hernia
Adhesions