I had this conversation with someone at work a few years ago. She was disgusted that taxes paid by people like her, who had chosen not to have children, subsidised people like me, ie parents. We were both on the same salary but my taxes were lower, plus of course I got child benefit and working tax credit. I was therefore better off, in terms of income, due to my lifestyle choice.
My argument was (and still is) that society needs new people to be born and educated and made into productive members. My colleague could be self-supporting all her life and retire fairly rich, but what good would that do her unless younger people - my children, their contemporaries and their children - provide her medical care, push her bath chair, and for that matter grow and distribute the food she expects to eat? I will have spent 20-odd years of sleepless nights, physical and emotional effort, dealing with falls and illnesses and tantrums and homework and heartbreak, having to work harder on "holiday" than I did in the office, can't choose where or when or even whether to pop off for a weekend, precious little peace or "me time", etc etc. I'm not saying it isn't worth it to me, love and company vs freedom and disposable income, that is a choice I made all right and I'm (mostly) happy with it. However, I am shouldering the vast bulk of the vital job of raising the next generation. All the childless members of society are expected to contribute is a barely noticeable fraction of their taxes. They're actually getting a bloody good deal for it IMO.
Like everything else taxes are for, it's sharing the burden. Because working parents do pay taxes too, you know. We're paying towards our own benefits, and those of non-working parents, and those of non-parents in need. We ain't parasites. We're doing a job like anyone else. Is it totally fair to have us do a full-time job of earning and a full-time job of child rearing, whilst others benefit from our labours and save their pennies? Only partly... And that, good friends, is why family benefits should continue, not just for the poorest members of society who may or may not be feckless (I'd argue that many if not most are far from it).
Let's not even get onto why rich people are assumed to make better parents than poor ones. If we lived in a meritocracy the argument might have some force, but we clearly don't. You will meet many capable, intelligent and decent people at the bottom of the ladder and some horrible oiks at the top. Being a parent and being a high earner are two quite separate skills so why should one depend on the other? I'd rather have a nice, sensible single mother who can't get a job at all bringing up the next generation of street sweepers (ie something useful) than one of those City financiers who teaches the kid how to spend other people's money like water and trample over the weak to get it. You wouldn't? Well, each to their own. I'm afraid that nice Mr Cameron probably agrees with you.