"Adolescents with a sexual interest in children represent an underserved population."
I don't know who needs to hear this but adolescents being "interested" in other minors is not surprising or disturbing. I suspect they might mean a sexual offender, in which case it would be simpler to just say that, rather than beating around the bush with a confusing euphemism.
Dealing with sexual offenders carries a certain amount of stigma, so sometimes someone is tempted to use lighter language like this, but it is problematic as it ends up conflating people who aren't engaging in criminality with those who have (guilt by association).
While doing research into one org, what stood out was how, basically, some therapists didn't want anyone to know that they had worked with criminals. I think that even criminals should have access to mental health care, so that is a bit concerning.
Other than that, there also appeared to be, well, unethical therapists, bad therapists who mistreat their clients, because they are criminals and there doesn't seem to be a whole lot of scrutiny of that. Naturally, they have some excuses for it but these are dubious.
It's what someone might call the "dark field". A sexual offender who hasn't been discovered yet (I'd be wary of conflating teens sexting with each other with that though).