Normally, people are able to see because the front part of the eye refracts light and points it to the back surface of the eye (retina). Myopia occurs when the length of the eye is greater than the optical length.  This makes it difficult for the eyes to focus light properly which is why distant objects appear blurry. The exact cause of myopia in children is not known. It does generally run in families and children seem to inherit a tendency to develop the eye condition from their parents. It usually develops in childhood and later stabilises when they progress into adulthood.
Aside from genetics, there are certain other factors which are known to cause myopia in children.  Premature birth may increase the likelihood that a baby or young child may develop myopia. The chances are increased if the baby has retinopathy of prematurity.
Also, certain eye conditions such as cataracts can also cause myopia.
Learn more about diagnosing myopia