It's tough to measure disposable income in any consistent way. Like if a household pays to send their kids to private schools because the local public schools are terrible, is that included?

No, that’s a luxury

Two minutes ago you posted "Childcare is a necessity".

Parents want their children to be safe, healthy, happy and nurtured. They will pay for services to provide that, and whether we label that service daycare, babysitting, school, summer camp, nannying, extracurriculars, etc doesn't seem like it makes much difference.

Parents need their child to be safe. Parents want their child to be better educated than local peers.

Childcare is a legal necessity. Private school education is not.

Public school is childcare. Private|charter|home school is education.

And sometimes it’s the opposite

How is money spent on childcare different from disposable income? Differentiating them artificially would fail to account for people who want children, but don’t, because they don’t feel they have the income. Which is a lot of the middle class at this point.

Childcare is a necessity.

A dual income household where one person’s income is going almost entirely to childcare is effectively a single income household and NOT in the same class as a dual income household without that cost pressure. I don’t see how that difference could be seen as artificial.

People not having kids or not buying a house because it is impossible are not more prosperous than in decades when the basics were more affordable but they have more disposable income if they realize there is no point in saving for now unrealizable expenditures.

Savings are included in disposable income

Right, I assumed we were talking about discretionary income. Why is a distinction between income and disposable income relevant given that taxes sometimes provide services that would otherwise be purchased and sometimes do not?

A person who spends all of their income on necessities is not in the same social class as someone who does not