The short answer
Do not compare salary with childcare alone. Compare the household's change in take-home income and costs, then add the longer-term effects on pension, employer benefits, career progression and future earning power. The immediate cash answer and the long-term answer may be different.
This question is often reduced to one painful subtraction: take-home pay minus childcare. That number matters, but it is not the whole financial result.
Build the calculation in two columns
Then include the human realities. One partner may want to return even if the short-term financial gain is modest. Another may need more time away. Flexibility, identity, health, care and family preference matter.
The fairest household calculation should not treat childcare as one woman's personal expense. It is a family cost connected to both careers.
The question is not only “what is left this month?” It is also “what options does this choice preserve?”