Firstly, the optician will ask about family history to see if there are any known problems with eyesight in the family. You will also be asked has your child any problems reading, looking at pictures, or seeing small objects.
The diagnosis will be made by by a visual acuity test where the optometrist will ask the patient to read letters from an eye chart from a specific distance. This will allow the optician to assess how clearly your child can see. He will also perform other tests which will measure how the eyes focus light and will determine the power of the lenses needed to correct the refractive error.
The optometrist will use a phoropter which is an instrument used during an eye exam to measure refractive error and determine what prescription is needed. The patient will be required to sit behind the phoropter and look through it at an eye chart. The optometrist will then change lenses to see which lens gives the best vision. 
He will also use a retinoscope which is an instrument which is used to shine light into a patient's eye. A retinoscope is used to establish eye co-ordination and how well the eyes work together.
If glasses are prescribed the optician will help you pick out a suitable pair for your child’s needs.
Learn more about treatment for myopia