Studies suggest that 5-10% of school aged children have an eye teaming or eye focusing problem. Most school screenings only assess whether a child can see 20/20. That measurement is of sight only and in no way an assessment of how the the visual system functions. In general, we are all born with sight, however, we have to develop and learn (binocular) vision. Most people develop efficient binocular vision skills on their own by outdoor/indoor play. During that important playtime, we are being exposed and adapting to varying sources of visual stimuli. There is however, a percentage of children that do not develop these skills on their own. These children are at risk.
As this position paper states, health care professionals, including eye care providers, more often use a problem-based approach to clinical assessment, making it easier to identify children who struggle. However, many children do not report symptoms, even though they may suffer from a binocular vision dysfunction. Complaints may not surface because the child doesn't know they see differently than everyone else; it's their normal. They assume their vision is 'fine' because they can see 20/20.
It is important for children to have a comprehensive visual evaluation from a developmental optometrist. It is crucial to determine whether a child's visual system is leading or impeding.