A key design failing of spectacles is that they slip down your nose and need to be pushed up again. Most spec wearers do this by applying a finger to the central part of the spec where it crosses the nose, but less experienced folk may apply the corrective finger to the main glass surface. This is ok unless aforesaid fingers are sticky with business of everyday life, which is only natural when aforesaid inexperienced user is three years old.
Where am I going with all this? Well, several years ago a Close Family Member narrowly failed some vision screening test at the age of three. The medical juggernaut kicked in and a few weeks later spectacles arrived. But the effect of this intervention was to reduce his visual ability because it converted a minor refractive error into a pair of specs which were always covered in various sorts of gub. One day I sat on the spectacles squashing them beyond repair. We didn't feel motivated to replace them and life went back to normal.
Here is a good review of vision screening in children.