Relevant part

Introduce a “negative child benefit” tax for those who do not have offspring. This may seem unfair on those who can’t or won’t have children, but it recognises that we all rely on there being a next generation and that everyone should contribute to the cost of creating that generation. Use the funds to fix the UK’s broken, expensive early-years care system.

So it’s not about taxing childrens but taxing people without children. Not that I’m in favor of those talking points.