How is intelligence in children measured? Intelligence is dependent on mental age and education. So how can an IQ test be performed to check the mental/intellectual capabilities in children?
There are specific intelligence tests developed for children in different age ranges, e.g. from 4 to 8+11 months years, and 9-16 years. The site mentions that their test 


  ...is a culture fair testing method. Test takers are equally assessed regardless of cultures, languages, backgrounds or education level. 
  Also, the test duration is 30 minutes, which is well designed to not only suit children’s attention span but also generate precise results.


Note that I am not familiair with this website and cannot verify their claim that 


  Our Phd-certified test was established in 2008. Nearly ten years since, the test has received approximately 16 million sittings.