I read that the "Toyfreaks" YouTube channel has recently been banned, and honestly I'm surprised it has taken this long
www.independent.co.uk/news/youtube-toy-freaks-greg-chism-videos-daughter-bad-baby-father-children-crying-throwing-up-a8068686.html
I took DD to a friend's house last year, and the children were watching this on friend's DC tablet. Honestly, I thought at the time that it was creepy and "off" and I stopped DD watching it. The dad gives me the shivers, he's very odd and has basically used his daughters as trained monkies,invading their privacy horribly in the process.
As well as this, you have the weird pregnant Elsa videos - if you like your conspiracy theories, Google Elsagate. Some people think that these cartoons are basically a way to brainwash kids, or a form of product placement. I wouldn't go that far, but some of them are really disturbing, featuring cartoon characters being murdered etc. All against a backdrop of nursery music.
I know children tend to like strange/gross humour, but these videos are all a bit weird, and frankly I prefer Bagpuss 
I know a lot of babies/toddlers who use tablets, and parents aren't always sitting right beside them to check what they're watching, and I suppose they see Elsa or Spiderman and think it's alright. Even kid's YouTube doesn't do a great job of filtering things out.
I'd link a few videos as an example, but my phone won't let me.