Dom, which one of us has been living in Poland for more than two decades and still lives here? It's not you, is it.
I lived there twelve years, eight years in Wrocław, until three years ago. So don't pull that $hit with me. 3500 PLN will barely cover the living expenses of a rather monkish single foreign male. I'm a rather monkish single male. With no savings. Hell, I'm a rather monkish single male, and I was spending more than 3500 PLN on living expenses alone.
A foreign couple with child would be living at poverty level on 3500 PLN a month. That's just plain ridiculous. Even at 6000 PLN, it's not going to be any party. Tolerable, yes, but not the high life, by amy means.
Last time I checked it was illegal for employees in Poland to be charged for work permits
Who said anything about work permits? Not I. They will have to pay for their residency permits, though. And their visas. And documents with translation. And airfare. And trips to and from the embassy. And hotels, if any. And shipping, if any.
If they get relocation expenses, great. But I wouldn't count my chickens until they are hatched. If the company does not give them an allowance for relocation expenses, then it has to come out of their income in Poland, one way or another.
You're very simply wrong there.
You're not going to do very much better than 2200 PLN a month all inclusive (rent, administration fees and all utilities except phone/internet/TV) for a one-bedroom apartment in Wrocław that's suitable for a family and with good access to public transportation. Anything much cheaper is going to be dodgy, have hidden expenses (like bad windows or an impractically small kitchen), or be very inconvenient in terms of transportation. And that's for a basic apartment, 60 meters, not anything luxurious.
A very good friend of mine
Everybody has "a friend" that got a good deal somehow. I have a friend who won big on the lottery. Again, these are exceptional cases and the OP can't count on such luck, especially as a newbie foreigner who doesn't speak a word of Polish and knows nothing about the city or the local culture.