How would you like words to look like this when you are trying to read? In the image, the words are not moving, but in reality, these words would be moving in a see-sawing motion.
A number of children that I see actually experience this distortion. The problem is that they don't know that everyone else does not see what they see, and they cannot understand why these other children can read but they can't.
So if your child is having trouble reading, ask them "how do the words look for you when you are reading?" If they say they look "normal", you need to say "but what is normal for you?" "I can't see what your words look like, because they are your eyes, and I cannot see what your eyes see".
This may or may not result in you finding out that your child is experiencing distortions, but it may elicit a response that indicates that what they see is not what you see.