I've seen all forms of porn, some more grotesque than others, but not once have I come across a site blatantly advertising underage porn.
Now, I don't deliberately look at gay porn sites (doesn't do anything for me), but if the term "teen" has any similar meanings for gay/bi porn as it does for straight porn, then it means the actors are 18 or 19. That's all.
Ads/banners blatantly promoting child pornography would bring all manner of fire and brimstone from the authorities, not only to the site in question, but to the owners of the ad servers. To that end, I really doubt such ads exist.