i understand how in the kid's sessions, the kids only had to be created once, in the pre-scratch session, so they didn't have to be created again in the kid's post-scratch session; but I'm confused as to why karkat had to create the trolls; sence his was the post-scratch session, wouldn't they have been created in the pre-scratch session?

The lack of ectobiology in the pre-scratch troll session was one of the reasons that their session was doomed to fail.