The real problem starts with the idea/wording of "one / two income family". It trivializes the issue and is likely rooted in the dated "housewife" concept that forces people into a permanent role, dependence, and general lack of choice, a social structure that really needs reform (and has been to a great extent in many countries).

Why can't parents work (earn) and take care of their kids at the same time? Why assign a (permanent) role of stay-at-home-parent to essentially half of the adults (with children) risking unnecessarily trapping them in roles and financial state that's unfair.