Road Safety - the information is taken directly from the CHILD SAFETY MANUAL produced by the Child Accident Prevention Foundation of Southern Africa

Young children have certain limitations:

Size: They are physically small and drivers cannot see them when they are in front or behind a vehicle. Sometimes they have to walk between parked cars to see if there is traffic.
Eye sight: Children will see what interests them when they look down a road and therefore will not notice an oncoming vehicle. They also have limited scanning vision. i.e. they see things directly in front of them and may not see things approaching from the side.
Hearing: Young children are unable to correctly identify the direction where sound comes from and may also not be able to identify the different sounds in the road situation.
Concentration: They can only concentrate on one thing at a time and for short periods.
Distractibility: They are impulsive and easily distracted. They may run into the road without warning. Movement of bright attractive things and animals distracts young children.
Limited experience and knowledge: their experience of traffic situations is limited and they are unable to understand abstract concepts such as safe and dangerous.
Unpredictable: their impulsivity and spontaneity makes them unpredictable.
Egocentricity: Young children think that the world revolves around them and that they are the centre of the world. So they do not always realize that there are other factors in the complex and dangerous traffic situations.
Prevention
Visibility: Parents must ensure that children can be seen, e.g. dressing them in bright colours; reflective strips on school bags.
Active practice sessions in the road with the child are necessary to apply safety rules that have been taught.
No child under the age of eight years should cross the road without the assistance and supervision of an adult. 
Cycling: Parents must ensure that young children under the age of 8 years, riding bicycles, are not allowed in traffic. They have not fully developed the physical and cognitive skills needed to display reliable judgment in the road situation.