I worked at Nickelodeon Online for three years before moving to Colorado, and as a website directed at children under 13, we were required to follow the regulations of the Children's Online Protection Act (COPPA http://www.coppa.org/comply.htm). Part of this regulation was that kids needed to be notified if they were being advertised to. Whenever we linked to an outside page that contained advertisers (as our banner ads for Transformers, movies, etc. often did) we would need to have a bumper page that would alert the user that they were about to be advertised to.
There were some ways around this, however, that my boss would sometimes utilize. To advertise one of the games we had on the site, employees would pretend to be children on competitor's message boards and talk up the Nickelodeon game with links back to our site. This guerrilla marketing campaign never alerted the children that they were in fact being advertised to, and also infiltrated a community that the children cared about. While I'm not sure whether or not the activity was legal or not, it was an unethical because it violated children's privacy and was approached in an inherently dishonest fashion. It's a good thing that there are protections for children online, and even if loopholes exist they shouldn't be exploited.