Just because you are not watching them, doesn't mean that they have to be glued to a screen. They could be exploring their environment or other things that engages their motor skills and other senses.

I wonder how many commenters here have children? A screen with moving images and sounds is absolutely hypnotic to a small child, and they will get stuck in front of it and sit staring for hours. When it is turned off, they will incessantly beg for it to be turned on, to the detriment of all other activities.