How can you have an adult consciousness in a kid's body anyway. Isn't your entire brain different?
I think it's likely the brain was, largely, unaffected. That's to say it was "wired the same" even if the chemistry was off due to the age of the bodies. You're right that the brain of a child is different that of the brain of an adult but that's all from decades of re-wiring as we age. The transported accident only removed the genetic-babble that made them physically adult. I doubt this would have impacted the make-up of the brain.
Preteen Keiko may have had slightly more unstable moods due to wacky teenage-girl hormones but by-and-large she'd be the same person.
As for the "situation" O'Brien couldn't be blamed too much for being uncomfortable but it's likely after time they would have adjusted and found a balance that made it work. Any physical encounters likely would have waited until Keiko's body matured more and O'Brien got over any societal ickiness over it. Society itself likely wouldn't give their situation a second glance considering the kind of world they live in. She could be in with aliens and all kind of goofy shit being possible, O'Brien could explain to any second-glancers with a "transporter accident." Looker: "Ah! Gotcha!" and then they nod at one another and continue on their way.