I'm going to split your arguments into parts, because they don't make much sense clumped together. I have a hard time understanding it.
> not whataboutism because it is valid to compare taxation regimes.
But this isn't a taxation regime. It's a penalty for choosing a way of life. It's as if the non-parent is responsible for the far future income of the state, especially in a state (USA) with no state-provided pensions; a state that doen't contribute anything back to the future non-parent compared to a future parent.
> the onus is on you to justify your claims
I already demonstrated that the only states that implemented it were totalitarian states. Are you in disagreement with this? Proportional taxation (fixed % of income) and even progressive taxation (higher % of income for higher income) is used everywhere, totalitarian state or otherwise. I'm not going to argue why that type of taxation is a good thing. Ask a LLM.
> (doing your job at keeping the birthrate above replacement)
Not my job. Not _a_ job. Not unless you consider that women should stay at home, cook and take care of children. BTW, do you?
> applying the exact same regime to another characteristic [...] Is totalitarian while applying it based on income is not
Sorry? Should that be the other way around to be comparable? More children, more tax? The state spends money on children, even in USA.
> This will incent the rich to have more kids because a 10 % penalty when you are making a million dollars a year is 100k but it's a rounding error if you are in poverty because the first almost 32k of household income is tax free if you joint file.
The rich have more disposable income. They won't feel 10%. A poor person is already at their limit. 10% will break them, so they'll have more kids and they'll be poor too. They will not contribute to future taxes. "useless turds" as you said - be careful what you wish for.
> the first almost 32k of household income is tax free
Oh, so they're exempt from childless tax too? That's moving goalposts.