Coming from a 40-hour norm, the scale of part-time work here reads as a lifestyle quirk rather than the outcome of tax and childcare economics. The follow-up question is a practical one: employees at larger companies have a statutory route to request fewer hours, and it comes with conditions attached on both sides.
Mostly tax and childcare economics: income tax is assessed individually rather than by household, formal childcare is expensive, and a four-day week has been culturally normal for decades, especially once kids are in the picture — the Netherlands has the highest part-time employment rate in the OECD, for both men and women, not just as a "mommy track." You can ask: the Wet flexibel werken (Flexible Working Act) gives employees at companies with 10 or more staff the right to request a change in hours, working times or location after 26 weeks of employment. An employer can only refuse a request to cut your hours for weighty business reasons — genuine scheduling problems, safety issues, or real difficulty covering the reduced hours — and has to respond in writing within a month, stating why, rather than just ignoring the request.