I've been reading and thinking about the comments on this thread and deliberately not commenting because I have mixed feelings about the matter.
But a thought that occurs to me when this topic comes up is that in our very unequal society, there are always going to be those who can't really afford one child, let alone two or more. However much anyone tries to convince me that we all have the same advantages and opportunities in life, I don't believe it's true - and the reason I say this is because sheer luck gave me an advantage as a child from a (what was considered then) lower working-class background. Without that bit of luck in my educational life, I doubt I would be in the comfortable position I am today, financially.
We rely on an 'underclass' of low-paid workers doing menial jobs that keep society ticking-over and I don't think that has ever changed or is likely to change much. So when we talk about 'not having children if you can't afford them', we're basically talking about the under-privileged - but they, like anyone else, also want to marry (or not) and have children. So I think, ultimately, we're in danger of suggesting that only the well-off should retain that privilege.
OTOH, I can't fathom why anyone struggling to make a living would entertain the thought of adding to that strain by having a child or children they simply cannot afford.
So this policy is clearly aimed at one demographic. It won't affect those, like Boris Johnson, who possibly now has sufficient offspring to create a complete classroom of his own lineage.
Another thought I had is that, if people actually sat down with a spread-sheet and worked out the cost of raising a child over a period of at least 18 years - from a purely practical point of view, how many would decide that it was an expense they just couldn't afford? Even before you factor in future world or national economic uncertainty and the unpredictability of life itself?
And when we complain about 'our' tax money being spent on raising other people's children who clearly shouldn't have had them because they couldn't afford them, directly or indirectly those children will in future be paying the pensions of the generation above them.
This isn't just an issue about economics or fiscal responsibility. I think it's more complex and involves the notion of whether or not we want a more equitable and fair society - and if we do, how we go about creating it. At the moment I don't believe this current government care one jot about such matters (and this is not an attack on Conservative ideology intrinsically), it's every man for himself and the devil take the hindmost. And the 'hindmost' will, inevitably, be blamed for being 'feckless' - but how many are, as opposed to simply being unlucky and not being able to do much about it... except to stop having children, obviously.