YouTube is basically a repository for videos which you can search for and download. As pointed out above they can be home videos, sports events, and so on , but obviously they can also be violent, pornographic, racist and every other undesirable term you can think of. A lot of the stuff there is funny, entertaining and 100% harmless.
But the question you have to ask yourself is whether you would let your child loose in a video store *which does not have the benfit of age-rating at a minimum* and not check what they watch ?
Try searching on a few keywords yourself and seeing whether you trust the content. YouTube do try to remove offensive content, but there can be a gap between what you consider offensive and what they consider offensive.
It can be fun and interesting, but I would be wary of unfettered access by children and I would definitely discourage (through education) them from uploading content to the site.
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