Alright... let me ask you this:
Let's say that right this moment, I agreed to sell you some dope. I don't have any in real life, but I tell you I do, and we make an agreement, and you come to get my drugs.
Could I be prosecuted for drug dealing, considering that I had neither actual nor constructive possession?
Yes.
The precedent has been that Crime of sale is independent from the seller actually possessing the good to be sold in any manner.
The undercover officer has committed an Offence of Sale. He didn't possess the children, he didn't intend to possess them, nor was he in the process of obtaining them. He wasn't even acting as an intermediary in the sale of children. But none of this matters according to precedent.
The undercover officer has committed a crime, and should be tried for it, just like any other citizen.
What we have here is a case of an undercover officer being above the law. I'm not creating a new controversy here. It is commonplace and controversial for undercover cops to be allowed to commit crimes in their line of work. It causes there to be inequity in the law, and that is why I am having a problem with this.