It's long, uninterrupted periods of staring at a very near object, usually lit artificially unless you have a pleasant reading space by a window. It leads almost invariable to nearsightedness more than just 'poor vision'. This effect is more pronounced in children due to the fact that their eyes are still growing and change shape much more easily. There have started to be myopic toddlers where this was previously a very rare genetics based phenomena.

My daughter does have some wicked myopia. The optometrist blamed her reading.

You should get her something like VoiceDream or SpeechCentral and load books on to that. I feel like it would keep her eyes more active