I can't speak as a parent, but I can speak as someone with decent
knowledge in advertising... and the Google AdSense system of ad
placement doesn't always work out the way it should.  The ads which
are placed on videos are based off of the text which appears on the
page the advertising is on, including the video description, tags, and
video title... but it seems that the video comments and related videos
are also considered, and this can lead to 'misplaced' advertising.
This isn't very useful in the eyes of the advertiser.