Say to them they need to read minute to massive letters in the wall. You can make your own Eye chart either by writing some alphabets manually or take some mags and have letters in different size, keep that mag or eye chart some 50 metre distance, make one of the kid eye closes and start practicing your kid. Tell them that they must saw how they feel when seeing the different sized letters. Ask them to answer did they find any trouble in seeing some letters. Sometime they may feel to find the difference and they may not even able to spot the difference, so don't force them.