I think the secret is to expose the person to multiple languages at a very early age.
Its like having kids: the first is an impossible leap, the second is much harder than anticipated, the third gets easier and after the fourth its just a big mass.
What I find sad is that though over the years I have learned and used languages with reasonable fluency, nowadays I pick up a book or hear something on the radio and I struggle to understand it. I'm stuck to the same languages I use every day. Those languages have still influenced my view of the world and understanding of the languages I do use but it still feels like a loss.
I remember reading the intro to a Berlitz language course written by Charles Berlitz himself (a polyglot master of a dozen or more languages, if I remember correctly). He made the statement that speaking two or more languages is seen as evidence of higher intellect. They went on to assert that it is actually quite easy to learn a language. In fact, anyone who is not an imbecile can do so, but learning a second language -- ah, now there is the rub. It's difficult, typically, because of the incorrect approach and the lack of integration. In order to learn a language, one must speak and hear it. It is insufficient to merely read it or "speak" it silently, in one's mind.
getting back to your analogy: I will quibble with a fine point. It is unfortunately incredibly simply to have a child. Even those imbeciles from the former example seem capable (even rather proficient!) at having many. The difficulty comes in raising a child well. And I think that was what you were getting at. :) That is no simple task. But after one or two the parents learn the necessary skills.
Berlitz also makes the observation that learning a third language is easier than the second, as one is now finding patterns and building on those similarities rather than focusing on differences.
Your logic is in good company.
I will point out that children who are exposed to two languages quite naturally learn both of them with only a slight delay over learning a single one.
I have 2 kids. Everyone I've spoke to who has 3 or more has said that the jump from 2 to 3 is as big as the jump from 0 to 1.
But note that the question is not if 3 kids are harder. They're obviously harder than 2.
The question is (for the sake of the analogy), if going from 2 to 3 is harder than going from 1 to 2 or 0 to 1.
We are talking strictly about incremental difficulty here.
Without providing any hard research to back my thoughts, I'd also surmise that it is difficult to transfer this onto the difficulty of learning a language. My thinking on this is that if for example you are learning another language within the same family, that is a different level of difficulty than learning a language in a new-to-you language family. As a German speaker, there are other Germanic languages that look very familiar to me, where I could take a reasonable guess at the meaning. On the other hand if I were to learn Mandarin or Russian, it would be a significantly larger task.
But all pretense of democratic rule must be jettisoned once the kids outnumber the parents.
From 2 to >2 was the biggest hurdle and the reason is simple: you have 2 hands with which you can totally control 2 kids. With >2 you need to master a different technic to control (read: keep 'm safe) under the most unexpected circumstances.
Edit: and we speak 2 native languages, our kids have grownup in 2 different countries. Sofar they pick up languages pretty easy and I attribute that mainly to not being afraid to learn & make mistakes.