Any eye test for a child who is too young to read is very useful. In particular it is useful to pick up a condition called amblyopia, where one eye works much better than the other. The child can and does see through the one good eye, but the poor eye is neglected.
This is a particular problem in early years when neural pathways are being established in the brain. The developing pathways will not include information from the poor eye. As such the child will not have the advantages of stereo vision, which leads to less effective motor function for the child. Also in later life should the child damage the good eye, there is no ‘spare’ eye to fall back upon. Amblyopic children are three times more likely to end up on the blind register.
Early life treatment is important, but to be effective it must be carried out in the early years while the neural pathways are immature. Treatment beyond the age of about seven is rarely effective.