If you believe that all sex outside of a heterosexual marriage is a sin, then you have ample reason to think adult-child sex is wrong without needing to look at details (assuming we rule out the case of child marriage). Much of what I write will not seem relevant to you.
Instead, I start from the perspective that any sexual activity between or among consenting adults in private should be legal. It also does not warrant any sort of moral condemnation. In fact, much of it should be actively celebrated as the expression of a joyful sexual desire in the ways that appeal to the individuals involved. It is part of the good life.
So what about children?
Many children masturbate from a young age. Parents are often aware of this around ages 2-5, after the child has the coordination to explore systematically and before they decide to be private about it. Many children engage in games of doctor -- often just showing each other their genitals, maybe occasional touching, but rarely for purposes of sexual pleasure. I think this is all healthy, and so do child development experts.
Once children start puberty, they begin to think about sex a lot. They typically masturbate more often and care about it more. They are fascinated by the gender they are attracted to and fantasize about them sexually. They flirt, "hang out", date and sometimes form intense emotional bonds. Some kiss, make out, have oral sex, and have intercourse. Anyone who thinks these all appear at the age of 16 or 18 is out of touch with reality. They happen much sooner for a great many young teens. A great many who don't do such things with partners wish they could.
I feel clear that none of this is abnormal, and certainly none of it should be of interest to the criminal justice system if it is consensual. I would strongly urge young teens to take reliable measures to avoid STIs and pregnancy. I would advise girls below the age of 16 or so to refrain from intercourse. But that is not a matter of the immorality of sex or the desire to have sex -- it is practical advice based on my best judgment of what will make someone happy in the long run.
I believe that childhood sexual interest is natural, and believe that children should generally be free to do as they please unless there is a good reason to restrict them.
With all that in place, the question naturally arises whether some children might want to have sex with adults. I think some young teens often will feel healthy desires in that direction -- a classic case is the young teen girl with a young man. I do not believe that any children who choose to engage in sexual activity with (willing!) adults should be subject to criminal penalties -- only the adults should be at risk.
Pro-contact pedophiles sometimes paint all anti-contact people as prudish and fundamentally opposed to all sex and sexuality. I am not.
Yet I still believe that adult-child sex is wrong, and that adults (usually men) who engage in sexual activity with children should nonetheless face criminal penalties. I do not want society to change to make this sexual activity acceptable. Stay tuned.