I'm not sure why I've never heard of this being done, it would be a good use of GPUs in between training runs.
EVERY youtube video?? Even the 9/11 truther videos? Sandy Hook conspiracy videos? Flat earth? Even the blatantly racist? This would be some bad training data without some pruning.
Part of the reason that kids need less material is that the aren't just listening, they are also able to do experiments to see what works and what doesn't.