when the doctor says the children have 6/12 visionfor both left and right eyes, what does that means?

Answer 1

This is the same as 20/40 vision. It means they can see the same small object at 20 feet that a "normal" eye can see at 40 feet. That is to say it must be twice as big.
6/12 is the metric equivalent of this.

Answer 2

chance of being blind.