Are you for real? The question is why would a Christian have a child, knowing full well that child could effectively decide to send itself to hell. The implication here, of course, is that it is better not to exist than to suffer eternal damnation. So why do it? This is far, far different than any kind of temporal decision offspring could make that would result in pain. No, a bad decision may cause drastic degradation of life, but by the Christian's estimation, all that will one day come to an end when the child (well, possibly adult) meets God and enters heaven. Not being a Christian has eternal consequences that can't be undone, ever, under any circumstances. Knowing that the child could choose "wrongly," why risk it at all? Would you have a child if you knew God was going to damn it?
In some forms of Christianity, but not in others.
My form of Christianity, which happens to be the largest branch of it (supermajoritarian, in fact), does not believe or teach that only those who are Christians in life have the hope of being acceptable to God at the end. "Be a Christian or go to Hell" is not Catholic belief. I suppose I could go on about what Catholic belief
is - and considering that Catholics make up about 75% of all of the world's Christians one could say that Catholic belief is the heavily predominant form of Christianity, the "default" position of Christianity if one is going to paint Christianity broadly and generically (as you do in what you have written).
So, to answer your questions directly, and to provide you with the majoritarian position of Christianity, let's go through your paragraph.
Christians have children out of love for the idea of having and raising children, out of love for spouses, out of love for the idea of continuity, and because sex feels good and is pretty irresistible, and children naturally result from it.
"Eternal damnation" is possible, but most people die with some unrepentant sin and proceed into Gehenna, which is Purgatory, where those sins are paid. Then they proceed to Paradise awaiting the end of the world, the coming of Christ, the resurrection and final judgment. At that point, those who have been cleansed of their sins in Purgatory whose balance of good deeds is positive, enter the City of God, come to earth, and live with God in happiness. Those whose sins are unexpiated, those in deep rebellion against God, those with deadly sins, are rejected and thrown into the Lake of Fire for utter destruction.
Now, before the end of the world, Revelation tells us that there will be considerable times of troubles and signs - which means that unlike those of us today who die, with sin, and have time in Purgatory to get right with God, those remaining in rebellion against God with all of the obvious miracles of devastation raining down at the end may well not get the chance to spend time in Purgatory. But then, whereas people today have doubts, with angels filling the skies, and dragons, and marks and all of that, the people in the end times who are resisting God are actively doing it against visible forces, like Pharaoh did. Not bumbling around in blind ignorance, like us.
When we have babies, and watch children grow up, we know that they will sin just as we did. But we know that God is merciful and forgiving. We hope our children do well, and that we too do well and are accepted straight into Paradise at our deaths. But we also know that God cleanses sinners in Gehenna, so there is always hope. We know, too, that true repentance even at the very end, turning to God at the end, will result in a favorable response from God. We know that because Jesus gave an example of that in the Scriptures, to the thief beside him on the cross.
So, when you say "Not being a Christian has ETERNAL consequences", that may be so, but it doesn't mean that whomever did not get the message in life - a Hindu in Uttar Pradesh, for instance - is doomed to the lake of fire at final judgment. No. When he dies, he will pay for his sins in Gehenna/Purgatory, and then pass into Gan Eden/Paradise to await final judgment. Will he be resisting the God he sees in Paradise? No.
The various minority sects of Christianity violently reject the majority view, but what I just wrote IS the bulk of Christianity, so when you write about "Christianity" generically, see to it that you do not take the view of some minority and claim that that belief is Christianity. What I wrote here IS the vast majority of Christianity, and if you have to pick ONE sect and state "Christianity says", then you must choose Catholicism, because Catholicism is three times the size of the rest of Christianity combined.
And Catholics simply don't have the fears that you enunciated in your question, because we don't believe the premises you set out.
Christians have children generally not fearing that their children will be going to hell. You baptize your children and raise them with knowledge of God. They grow up and sin, and struggle, as all do. They either come around in their old age, repent, confess, are absolved and proceed directly to Paradise at death, or they die in an imperfect state, and attain cleansing in Purgatory, then proceed to Paradise to await final judgment and the City of God.
It's ultimately up to God, and God is kind, merciful and loving. Also just, which is why there is Gehenna/Purgatory.
Most Christians don't worry about hell, and don't believe that everybody who isn't a churched, believing Christian will go to Hell. Some do. They are the minority. You certainly can address the minority if you'd like. What you cannot do is ignore the majority view of 3/4ths of Christianity and set that aside, and call the opposing view of less than 1/4 of Christianity (many of the other Christian groups also believe something similar to the Catholics) "Christianity".
Certainly if the minority view turns out to be true, there is cause for great pessimism, of the sort that might even suppress the birth rate. But the majority obviously don't believe that at all, and therefore have none of the fears that your question refers to.
We have children for love, because of romance, and because family is good, and we expect that our children will work out their salvation and rejoin us in Paradise, sooner or later, and ultimately in the City of God. That is what Christians believe - if you have to use one word to refer to all of us.