She was a student at the time she received the letters claiming 'missing information' that led to the (unjust) claim. This does not mean she didn't have a job before that or some other occupation. She was around 30 at that time, and study was just part of her career path.

If you have several children, the child care allowance adds up — I receive over €1000 a month for one preschool child. Multiply this for more than one child and several years…

It is a system that is increasingly seen as hostile to citizens, because you need to update your registration with every change in income. It's silly too, because the allowance you receive goes to a child day care centre or kindergarten anyway, so you are just shovelling money around which leads to almost all parents being burdened with something completely avoidable. The answer is to just make child day care free for all children and not bother citizens with subsidies to pay it. It's probably cheaper to do it that way (we would be following in the footsteps of other progressive countries).

How often does income change?

I don't see what's 'hostile' about asking people to fill in a form a couple of times per year (at most) for €12k.

It changes every time your salary changes and when you take unpaid leave, even if only for a few days. For a dual income household this is already a bother because of the details you have to enter. For households with lower incomes it also doesn't stop with child care subsidies, but also a health care, rent, and an additional child specific subsidy to manage. Now consider that many people in lower income brackets do not have a fixed salary but one dependant on hours worked (think retail staff etc.), and you can almost guarantee that you will receive too much at some point, which will be reclaimed.

Compared with the ten minutes filing taxes takes here (with everything prefilled and rarely a need to change anything) this system is a high-risk travesty that wastes a nation's time.

€100,000 = ~$108,000

So if that's the credit for 3 children, for 4 years, that's ~$750/month. I have zero idea about how much it would cost for the Dutch, but in the closest city center to me in the US, the average cost for a licensed daycare center is ~$1300/month.

Not unrealistic, really, unless I missed something.

210 euro a month for 2 days a week supposedly https://www.xpat.nl/living-netherlands/kids/day-care/

That may well have been including punitive components. They're really good at that.