Lots of people have addressed the emotional side of it, but I'm also finding the idea that having children isn't expensive quite laughable.
As a nanny, your salary will have been paid out of your employer's post-tax salary, which is then taxed again to pay your contributions. Your employers are essentially earning to the tune of two full time jobs in order to pay you what is presumably a reasonable wage, as qualified 1-on-1 childcare. And whilst having a nanny is probably one of the more expensive ways of having childcare, I don't know many places that you get much change out of £1,000 per month per child for FT care, more so around us.
That's of course not including entire wardrobes for each child, each season (as they do grow). And food. And a slightly larger home because they do need a bedroom. And anything you do, even if it's relatively cheap, has to be multiplied by the number of participants - so a cheap ticket to something is suddenly £100 without any drinks or snacks. Or a 'cheapy' holiday abroad in school holiday times at £250 a head is still a grand for four of you.
Every single thing you do, multiply it by the number of people in your family - that's how much it costs. And you won't earn more. As a woman, the statistics say you're actually likely to be earning less. So your currently perfectly lovely lifestyle for one becomes absolutely without non-essentials because there's no money for them.