(The book is on how to make flash cards.. nothing to do with children older than 2 years old)
For example, at 6 months old, the text size should be 3", and gradually reduced to 2" and eventually 1.5" approaching 2 years old. By 3 years old, the print can be 7/8" print of the book, but if the child is less than 2 years old, the suitable size is 1" or 2".
I tried to find some research data on text size suitable for children between 3 to 8 years old from internet but not successful. Using the data above, I plotted a graph. From the graph, it seems to me at the age of 4-5, the size of the print has no significant difference. However, from my observation, I believe eyesight of a child will be affected not by font size, but how long do they read per day without rest, how far they hold the book, whether there are enough light, etc.
I think the fact that kids watching too much TV have higher tendency to wear spectacles demonstrates to us that we should watch out and not let our children reading continuously too long without rest. However, how long should each reading session should be? Without parental control, I am sure my kids can play with iPad, watch TV or read his favorite books non stop. That's not good. The fact that books tends to have lesser effect then TV is because it's less likely they can read non stop compared to TV, iPad or computers. But that does not mean they should be encouraged to read too extensively.
Another important thing is outdoor activities. When we bring our kids outside, they will tend to focus further and look further. I believe that helps. But how much? I don't know!
I do not have the answer on what's extensive enough to have impact on their eye sights. If you know any data, please share with me! But I would encourage my children to rest, look far from window after reading or using computer for 30 minutes. And if possible, bring them to play outdoor at least few times per week.
