But people who get kids already receive various financial benefits from the state in many countries. But it is usually framed as giving them money, instead of taking from everyone else. In the Netherlands you literally get money per child.

So if I were to reframe this tax as: “people with children get a tax deduction on their pensions”, nobody would bet an eye, as similar benefits are already in place for them. But where does that money come from? Right; Tax payers. So, to give people with children a tax deduction, everyone else needs to pay more. Financially, you end up in the same situation as what is proposed here.