I think the article is this one here.
It is a sexist article because it focuses on mothers paying for childcare and assumes that the costs come out of her pay.
However, more broadly, I get rather tired of the 'it's just a simple mathematical exercise' argument as to childcare and the lower earner.
Firstly, I'll just stress that this is about families who have choice. Who can pay for the childcare they need and the total family income is enough to live on. for a large number of families, there is no such choice, one way or another. We are basically talking here about professional women with careers.
But for those with choice, if you are just taking take home salary, deducting childcare and making your decision based on that, you are doing yourself a disservice if you describe that as a mathematical basis for the decision. Even if you want to do a purely maths based approach, you are failing to factor in pension, long term earnings and career potential. Whilst you may work at a loss for a while (though this article doesn't suggest most women do), the lifetime gain is likely to be huge. So you are not making a genuine management accounting decision.
More broadly than that, a pure maths approach only really works if you don't want to work, and want to see where you stand financially. If you do want to work, why the hell should the second earner (statistically, in a patriarchy, more often the woman) be required to justify their earnings in this way (again, assuming that the family can live on what is left either way). If you both want to work, a more honest approach is to deduct childcare from total pot and think of it that way. Too often this 'costs against lower earner' approach results in women feeling under pressure because they are 'selfish' if they work when they aren't really adding anything to the monthly pot.