Come to think of it...
If an Ocampa can only have one baby, why doesn't the population of Ocampa cut in half every five years?
In our universe: Bad writing!
The same as the silly nine-year lifespan.
In the Trek Universe: Maybe they had twins, triplets or more. Note that in "Dreadnought" Kes states that she had an uncle named Elrem. How could she have an uncle if the Ocampa could have only one child?
As for Neelix, I think that this
relationship should have been possible after Kes dumped him: