Here is a fascinating YouTube video showing the work of Nicola Farmer. She is shown working with children in the age range 6 to 12 years in a small private school in Essex, U.K.
Children seeing while blindfolded:
 

 
We suggest that you conduct simple experiments and use a good quality blindfold as in the video. The following ideas might be useful:

• Different coloured pencils, using basic colours: red, yellow, green and blue.
• Simple shapes such as circles, triangles and squares.
• Holding up a number of fingers.

If the child is able to do these with a high degree of accuracy, then move on to identifying numbers and letters.
Another idea is to put a picture in front of them and see what they can identify.

It is important that the children are never put under any pressure. Keep things light and, above all, have fun!
It is also best that the sessions only last a few minutes and that it is explained this is quite a natural ability.