As long as they don’t do it on real humans nobody is hurt.
Living out the fantasy of having sex with children (or other harmful sexual practices and fantasies) wit AI or alike can strengthen the wish to actual do it in RL. It can weaken the strength to abstain. If you constantly have fantasies where for example “the child AI wanted it too” then it can desensitize you and making it harder and harder to push that thought aside when in a tempting situation. Instead of replacing the real thing with a fantasy you are preparing for the real thing. Some pedophiles already interpret children’s behavior as sexual that isn’t at all, but the AI might be told to act in that way and strengthen these beliefs.
This is still something that is and has to be studied more to fully understand it. Of course this is difficult because of the stigma. There might be differences between people who only are attracted to children and ones that are attracted to adults and children and there is just not enough data yet, but even the communities in which pedophiles who do not act on their attraction discuss coping strategies this is heavily discussed and controversial.
If you are interested in the subject a bit more, this is a start: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8419289/
I am so far behind on series on Netflix & other streaming services, that I can watch the next 10 years only ones that made it to a reasonable amount of episodes and seasons and got a nice wrap up. I am not investing my time in new ones that might not make it. I learned my lesson with Firefly.