I suspect the scenario looks something like this:
-Harassed parent sticks an episode of Peppa on YouTube to be able to, I dunno, use the loo in peace.
-Small child clicks on some of the related video links a couple of times.
-The internet being what it is, three or four degrees of separation is more than enough to end up somewhere dodgy.
(I will confess to once having stopped my kids from following the link to a 'Peppa Pig discovers bacon!' animation while they were watching episodes on YouTube...)