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:
Samantha Wildman had a husband.
Niomi wasn't born out of thin air.
That would have been a great homecoming: "Hey Dad, Mommy was cheating on you with a Hedgehog right in front of me, knowing you were faithful waiting for us to return all these years."
Love conquers aaaaaaaaaaalllllll!
Seriously, it would have explained why Neelix was hanging around the Wildman family as much as he did without making him look like a shady figure.
And I still think they would have been a lovely couple.
I thought he hung around them cause they were the only family only Voyager?
Didn't he compare himself to Niomi in "Once Upon a Time" and implied he looked after her because he didn't want her to know what's it's like not to grow up with without parents? He was trying to be her surrogate father. It wasn't sexual with Samantha as much as he was just lonely. Everything Neelix did was a act of desperation not to be alone again.
However on the debate about how many children the Ocampa can have, I agree with the point you brought up about Kes mentioning she had an Uncle. Yes, that would imply they are capable of having more than one child at a time.