So, the cost for a typical 4 year-old is about PLN 400.
Where? Certainly not in many places. In fact, in many places outside of the major cities, 400PLN will get you a private nursery. In fact, I'm familiar with one public nursery - and the highest cost paid is 180zl a month, plus food (which is about 6zl a day for everything).
In reality, teachers are faced with a nightmare of bureaucracy as each kid has to be timed in and out - each hour is charged you see - and parents have to declare their requested hours of 'childcare' for the month ahead.
No they don't. Have you actually been in a nursery, or are you basing this all on some hysterical news report?
Incidentally, what's wrong with declaring up front? Nurseries are often owned by private owners, who need to be able to issue invoices for payment - and no nursery in Poland is going to take a kid on post-payment terms due to the general Polish mentality.
Now you need to be well-off to be able to afford it
I fail to see how 400PLN (truth is, it's usually quite a bit lower) a month is "unaffordable" to someone who works full time in Poland.
[Private pre-schools (often poor quality) charge PLN 1000.]
Some do. Plenty of others don't charge so much. In fact, 1000zl usually is only found in decent nurseries in nice locations in big cities. Again - the vast majority aren't paying anything near this.