My son has only recently discovered YouTube, but his interests are restricted exclusively to those videos which further his obsession with maths: Numberblocks mainly, as well as kid-friendly explorations of math-related phenomena, which he adores as they fly over his head.
We’re sanguine about all this, because we’ve done the sums and are prepared for his eyes to go a little square if it allows him an hour a day to refuel his list of maths questions, rather than repeating the same ones to us, ad nauseum. But mostly, we’re hoping it will rub off on his little sister, who has yet to take to screen time at all.