Couple A: -$100 + $200 = +$100
Couple B: -$100 + $0 = -$100
Historically it was considered a beneficial necessity to gather the children to write down knowledge so that it could be brought back home for the whole family to learn from, but in the age of the internet perhaps separating children and parents is never good at any (young-ish) age?
It gives massive structural advantages to couples with low income, in the form of a lower marginal tax rate. Does it really discriminate between single and dual income though? I wasn't aware of that.
I think it's less "mindsets" that have changed so much as the incentives themselves. People no longer need to have kids in order to have sex or to have a comfortable retirement, so many simply don't. Though I'd agree there's certainly a mindset shift that has developed along with that.
Since the advent of air-conditioning, there really isn't any good reason to close schools during the summer. But, like the internet bit before, we've just never bothered to stop and actually think about what we're doing. We carry on with the status quo simply because that's what we did in the past. Not because it makes sense, just because that's what we do.
But in establishing subsidized daycare now, we don't have to think about the time before air-conditioning was invented. We only have to worry about the constraints we have today. Hot summers are not a practical problem as of right now.
Let's say you made 150k and partner made zero. If you were filing as single, high tax bracket you would get is 24% at everything past 100k. Married filing jointly, 22% at 94k. Also, you could borrow your spouse standard deduction as well to help reduce the tax requirements.