It is NOT adequate to evaluate a child's visual development based merely on the distance vision test (or we normally call it the '20/20 eyesight test').

It is also essential to take notice of the following pointers:
Acuity-Distance To check if the child can sustain a clear vision of objects 20 feet (6metres) away.
Acuity-Near To check if the child can sustain a clear vision within reading distance.
Ability to Focus Ability to focus on objects from varying distances.
Eye-Tracking and Fixation Skills Ability of the eyes to look at and accurately follow an object. (Eg. The ability to move the eyes across a sheet of paper while reading)
Binocular Fusion Ability to use both eyes together at the same time.
Stereopsis Ability to tell depth and perspective.
Eye-Teaming Skills Ability of the eyes to work as a coordinated team.
Hyperopia (Long sightedness) Whether the child experiences better vision for distant objects than for near objects.
Colour Vision Ability to distinguish colour hue.
Reversal Frequency Whether the child mixes up certain similar-looking alphabets or words, such as: b,d ; p,q ; was/saw.
Visual Memory Ability to store and remember visual signals.
Visual Form Discrimination To check if the child can differentiate between different shapes, colours, sizes, positions or distances.
Eye-hand-body coordination Ability to combine visual input with hand and body movements.
The visual skills listed above contribute significantly to a child's success with reading, school achievements and social life. Therefore, it is advisable for children of age 3 and above to go for regular eye check-ups (preferably once half a year) with the optometrist. Regular check-ups can look after developmental milestones, spot potential vision defects and manage them immediately.

As a parent, there are many things that you can do to help your baby's vision develop. First, it is important to understand the different stages of eye development:

New born babies The structure of a baby's eye doesn't differ from that of an adult's, however, because it is smaller in size, most babies are far-sighted. At this stage, your baby should already be able to detect light, he can also blink and roll his eyes. However, since the corneal is not fully developed, your baby only sees in blur patches of black and white (within a few inches).
4-months old Your baby starts to gain a clear vision of objects within 5 feet. He begins to follow moving objects with the eyes and reach for things. Hand-eye coordination, depth perception, colour vision and visual memory begin to develop.
6 to 8 months old Your baby should begin to turn from side to side and use his arms and legs. Eye movement and eye-body coordination skills should develop further and both eyes should focus equally.
8 to 12 months old Your baby should be mobile now, crawling and learning to walk. As his depth-perception develops, he judges distances with greater precision. He can now play with smaller toys (such as Lego) and he is beginning to describe things that they see.
Above age of 1 Your child's eye-hand-body coordination and depth perception will continue to develop. At the same time, visual acuity becomes more accurate. The development of the eyes will last till about 6 years old, before maturing.

During the period from birth to the age of six is the most crucial time for eye development. Within this period, your child needs to be continuously exposed to bright and clear visual images, to stimulate development. Any possible development problems, such as congenital cataract, dropping lids, etc., will hinder growth and cause amblyopia. Secondly, if there is a huge difference in degrees of the both eyes, it will directly cause adverse effects to the development of the brain. Thirdly, children with strabismus, will face difficulties in eye-teaming and coordination developments. Lastly, if the child experiences nerve problems near the eyes and the visual sensation area of brain (such as trauma), it will also affect the visual development of the child.

Therefore, under any circumstances, parents should not disregard any complains about visual problems from their children. However, under situation where only one eye has visual defects, it is usually very difficult for the child to identify the problem. Hence, parents and teachers should take the initiative to observe the kid's behaviour and the appearance of his eyes. As for toddlers between age 3 to 5, they may have difficulties expressing their complains even if they are able to detect visual problems. It is advisable to bring your child for a full eye examination every 6 to 9 months.

The following are some symptoms to take notice of:

Abnormal appearance of the eyes:

ĦE Drooping eyelids and cover pupils.
  ĦE Difficulties in focusing both eyes.
  ĦE Blood vessels around the conjunctival turn red, sore eyes with thick secretion.
  ĦE Eyelashes grow inwards, touching the cornea.
  ĦE Unstable vibration of the eyeballs.
  ĦE Afraid of light and excessive tearing.
   

Abnormal behaviour:

ĦE Frequent blinking and rubbing the eyes.

  ĦE Frequent squinting.
  ĦE Has a habit of reading and watching TV at a close distance.
  ĦE Experience difficulties in coordinating both eyes. Often skip words while reading.
  ĦE Like to turn or tilt head to use onlyone eye when looking at objects.
  ĦE Often takes a long time to complete homework and uses finger to maintain place when reading.
  ĦE Find difficulties looking at the blackboard and copying from it.
   

Complains:

ĦE Headache, acute pain and itching of the eyes.
  ĦE Blur vision (despite of distance).
  ĦE Sees multiple images.