Scrap the Ocampans living only nine years
Maybe one of the easiest ways to make the Neelix and Kes pairing less gross would be to remove that nine-year age window completely and just let Kess be in her 20s. Still not the best thing in the world for her to be dating a man in his 30s, but it's far more socially acceptable than a 34-year-old and a two-year-old. So, you know, progress.
The age of the Ocampans didn't help Kes as a character or the Ocampans as a species. It made them actually pretty lame. With an infinite number of ideas you could have given the Ocampans, the writers made them age fast. Was this some weird fetish that the staff writing room had? One can only hope not but it was never going to leave anything but a gross and unnerving feeling in any of our brains.
You could have made the Ocampans dependent on only water. Maybe they can hear so well that they need to wear audio suppressors just to survive outside of Ocampa. Maybe they're the Delta Quadrant Betazoids, unleashing pheromones that drive people wild. Or, maybe you make them synthetic robots. Literally, anything would have been better. Granted, some ideas are not much better, but still, better.