A room for each child and an office is not exaggerated in my opinion.
Yes, if you only have 150 sqm in total, I find that exaggerated. But in the end, everyone has to decide for themselves how they want to allocate the space. I would never want to live with an 8 sqm kitchen for 5 people.
Construction prices are just high, but does that mean you should completely give up? What is the alternative for you? These days, it’s hard to find an apartment that size and there is hardly any price difference either.
The alternative would be, for example, two children sharing a room and the office upstairs. PS Your guest WC doesn’t work.