I guess the GP's suggestion is to let people think twice before getting any? Children are expensive without this tax but most people don't think 1 second about that because it is somehow a right to have them so the state will provide (and in many countries here, indeed they will). If you can clearly see that you and your better half will never be able to support them until they are 18 (which goes for many people already, without any ridiculous taxation), you might think twice about having any at all?

If you want to go to jail for many years and screw up your life significantly, possibly never be happy again, perhaps.

Is this supposed to be any kind of counter argument? "Parents could simply kill their children."? Seriously?

It was supposed to make it obvious that your tax proposal is absurd. If you don't plan to make murdering children legal, then what you are de facto proposing is making some people slaves. From X children forward, poor people become slaves, having to do forced labor just to pay the "child tax" that they can no longer avoid.

If you think that there are circumstances that justify slavery (they should've thought about it before having that many children!! It's a reasonable punishment for their deeds!) then, sure, I guess.

That is quite a far fetch you are making there, from implementing a financial penalty for not taking responsibility for ones actions (getting more than one child) to "slavery". I think you are mispainting or misinterpreting the picture in an extreme way. No one is suggesting slavery here. That's a strawman,

And yes, people should think before putting a child into this world. I don't know how that is even debatable.

But, how is that not the logical conclusion?

I agree on "people should think before putting a child into this world". Will you agree also on the following:

* Despite the should, some people will not think before doing X. So the punishment you propose will not just be an abstract thing that convinces people to "not do X", it will be something you actively inflict on people (otherwise it's completely meaningless, if not enforced).

So let's go back to the tax. Statistically speaking, who are the people who have lots of kids? Those who are rich (and might be deterred by an additional tax) or those who are poor and can barely feed said kids? Now, if you enforce an additional "kid tax" (and not just as a temporary thing, but as a perpetual lifetime punishment), what exactly is the effect that you think it'll have? Won't you de facto withdraw or limit "social security" exactly from the people who need it the most?

Just look at the data. Population doesn't increase when people become wealthy, on the contrary, it decreases. Rich people just don't have that many kids. Your tax could only serve the opposite purpose, while making the poor even more miserable.