Causes of squints :

The exact cause of a squint is not always known. Some people are born with a squint and others develop one later in life. Sometimes they run in families. In children, a squint is often caused by the eye attempting to overcome a vision problem,
     such as :

short-sightedness – difficulty seeing things that are far away
long-sightedness difficulty seeing nearby objects
astigmatism - where the front of the eye is unevenly curved, causing blurred vision
Rarer causes of a squint include - some infections, such as measles, some genetic conditions or syndromes, such as Down's syndrome, developmental delays, cerebral palsy, other problems with the brain or nerves. A squint can also sometimes be a symptom of a rare type of childhood eye cancer called retinoblastoma. Take your child to see a GP if they have a squint to rule out this condition.