This. Not to mention, how will this be judged how to tax childless people more? Will every childless person be taxed the same? Will couples be taxed more? When do you start taxing people this additional amount - when they turn 18/leave full-time education? (Great way to encourage teen pregnancies!). When do you stop taxing people? At retirement age, or when their own children turn 18, or leave home?
What about step parents? Parents who adopt? They haven't had children themselves, so haven't actually increased the population number, but they are the ones bringing the child up (and facing the costs of having to do this). Will the birth parents then be charged the additional tax? (I fear for those children who need to be taken into care, in this situation, if birth parents don't want the extra tax.)
Not to mention it's just a kick in the teeth for those who want, but can't have children.
As a childless adult (not through choice, we're struggling to conceive), I do not support this. What I would support, is the taxes that I currently pay being used to increase child benefit, or being used to assist parents more with ridiculously high childcare fees, or being used to increase statutory maternity/paternity pay.