there are actually many youtube channels which are brainwashing kids . you can not keep eye on their activities on internet always.. being web security expert i can tell that there are many ways to hide your identity and erase all footprints of things you did over net..
the best way is to educate your kids well and make their minds well developed that they can decide themselves what is bad and good for them .. and you can always guide them when they are not sure about things.