The persons perception of what is going is what counts here. The fact that the child isn’t actually a child isn’t relavent, only the suspect/defendants perception.
By the logic above, the following stings would also not work.

: Pandering. An office pretends to be a prostitute in a string, and arrest johns for pandering. It’s irrelevant whether the office is actually a prostitute.

Officer poses as a drug dealer. Supplies defendant with a substance that is not drugs (i.e oregano for marijauna, etc.) It’s irrelavent whether the officer is a drug dealer, or the drugs are real.

etc, etc…The police according to legislative and case law are permitted to use deception with criminals and suspected criminals. The line they can’t cross is entrapment, where they coerce the suspect/defendant into committing the crime, or make it so unbelievably tempting the defendant couldn’t reasonably be expected to refuse. Something like the police leaving an armored car open with bags of money lying about across the street from the parole office would fall under this heading, or threatening a person with any consequence if they didn’t commit the crime.
Fake drugs in a sting, fake money, fake hookers, fake children. The burden is on the defendant to prove that he knew that it wasn’t a real child, drugs, etc, etc.