I'm not the parent but if I had to guess, I'd say it's the way new media seems to erode our ability to concentrate for long periods of time. YouTube et al provide us with endless stimulation such that we never allow ourselves to get bored anymore. Then, if we're doing something we may find tedious and difficult (say, grinding problems from a math textbook) then we can't handle the lack of stimulation and get restless as a consequence.

Taking a break from all of that stimulation is a good way to quiet our minds and relearn how to focus.