I'm thinking your definition of paradox is quite different from mine. One of the best paradoxes from the mouth of Jesus is where He said,
"Whoever seeks to save his life will lose it; whoever loses his life for my sake and for the gospel will find it."
Paradox requires, by definition, an apparent contradiction that turns out to be true and non-contradictory. Exactly what is contradictory about Christian families having three children, two children, twelve children, or no children at all?
Your use of the word paradox reminds me on people who use the term ironic indiscriminately.
"It's ironic that we had pancakes for breakfast two days in a row!"
The word they should use, more often than not, is coincidental.
That issue aside, let's talk about one drawback in having a passel of kids. First, there is no guarantee that each child is going to spend eternity in heaven with his or her parents (with all the saints, and Jesus of course). As my pastor is fond of saying,
"God does not have any grandchildren!"
In other words, each generation of children (and hence, each child) has to make up its mind to follow or not to follow Jesus. It's great when they do; it's tragic when they do not.
I'm not saying we should therefore refrain from procreating out of fear of giving birth to a soul who may spend eternity in hell. I am saying there are no guarantees that our offspring will become believers in Jesus Christ. Would to God it were so; sadly, it will not be so.
You talk about "fundamental levels," which is good. One fundamental that screams from your question is that life--every life, even the life of a baby in utero--is a sacred gift from God. While not making light of a woman who conceives as a result of rape, even the life within her has a right to be born. For her to abort that baby would not, however, be the unforgivable sin, in my opinion.
There are certainly indications in the scriptures that children are a blessing from the LORD. Moreover, the psalmist talks about the blessedness of the man whose quiver is full of arrows (i.e., children; see Psalm 127:5). Then, too, while the LORD's command to "be fruitful and multiply" is still a good command, it's just not as crucial today as it was when the entire population of the world numbered exactly two: one man and one woman!
In conclusion, I agree with you that having children is a good, albeit sobering, thing. This life we have is not all there is; there is a life--or death--to come, and for this reason each Christian couple should have only as many children as they think God wants them to have. For some, that number will be none; for others, 12; for others, only foster and/or adopted children; for still others, 2.5 (!)--I'm talking averages here ("lol," as the young people say nowadays). Your suggestion that we all have as many children as we possibly can may be well intentioned, but it misses the mark in both discernment, wisdom, and balance.