Something like Sesame Street might be good. My problem with YouTube is it creates feedback loops where if you like something you seek out more of it, but with a near infinite range of topics. Throw an underdeveloped mind in there who is using basic stimulus to work out what is good and bad and who knows what will happen.

Wouldn't surprise me if that feedback loop was damaging even for many adults, but at least they are responsible and already have some grasp of how the world should work.