Okay, I guess there are two issues: the first is that pedophilia is completely unnecessary, if the goal is just to make this dude look worse. We know he's a bad dude, Arya knows he's a bad dude, EVERYONE knows he's a bad dude. We don't need anything "extra" to make him even worse.
But, when that's brought up, the retort is "well, it's going to play into his assassination scene". Which, okay, fine--they need to establish that he's a pedophile. What they didn't need to do, was trot out a terrified child and then have Trant indicate he's going to rape her so horribly that she won't even be rapeable ON THE MORROW.
Shit, instead of saying "Yes, we have a girl like this", tell him "Sorry, we don't have any children to rape here" and then he's pissed off, goes back to his quarters or whatever. Arya can still use the pedophile angle and we can skip Trant drooling his pedophilia.
Would have been much more interesting if Trant was taking a vacation and just needs some time to breathe away from King's Landing, like a normal human being, and Arya still decides to kill him.