Most common cause of pseudopancreatic cyst in children is -
Pancreatic pseudocyst follows acute attack of pancreatitis and trauma is the most common cause of acute pancreatitis in children.

Most common causes of acute pancreatitis in children include:

Blunt abdominal injuries,
Mumps and other viral illnesses
Congenital anomolies
Chloledochal cyst
Annular pancreas
Drug induced pancreatitis
Traumatic pancreatitis