You would think that, by now, the good and neutral churches (and the lawful evil ones, too), would have developed protocols for that kind of thing. You know, things based on spells like Dispel Magic, Dispel Evil, Discern Lie, Zone of Truth, Detect Magic, Sense Thougths, Detect Alignment, Dominate Monster (followed by ordering the creature to tell the truth), Dismissal, Banishment (if it disappears, it wasn't a real kid)...etc.
I mean, succubi and clerics/paladins/other religion-based classes have both existed for centuries and even thousand of years, the succubi have probably tried that scheme a million times and by now the high-ups of the church probably think when hearing about the issue "another succubus pretending to be a kid, I guess".
A more effective way to use its power would probably be to pretend that she's a Doppleganger courtesan and offer her customers to satisfy all their kinks, slowly and subtly encouraging them until fully corrupting them:
"There is nothing wrong if you feel attracted to your brother's wife, he will never know that you are bedding somebody who looks just like her".
"There isn't nothing wrong with it, I look like a sixteen years old, and those are legal in this kingdom".
"No, I swear this shape isn't like that of a fourteen girl, I can show you how a real fourteen naked girl looks if you want it"
"Have you ever considered doing it with a Catfolk girl? That's hot!"
"What if I take the shape of a Succubus?"
"A Succubus with your sister-in-law's face?"
"A Pixie would be really kinky, don't you think"
"What about a mermaid? or a lamia?"
"What about your cousin?"
"You know what would be really, really, really kinky? to do it with while I look like your real, blood-related sister! Come on! It's not that bad, she's seventeen and it's just a game, it's not as if I really were her."
"What about a halfling girl"
"I assure you, this is the face of a halfling girl, not of a nine years old child".
And them she could subtly pull the strings so he really gets the chance to make real some of the less offensive fantasies. That way, when she mind-controls a real kid and sends it to the priest/paladin's bed pretending to be her roleplaying a kid, and arrange for him to be discovered, the inquiry will reveal his perversions and give credibility to the idea that he's a child abuser.