As has been mentioned, people from the lower classes tend to swear more, as they do in America, England and elsewhere.
This is true, however it occurs far more within the Polish language. If you walk round town here, in the daytime, and count the number of times you hear the word "kurwa"...
Worth remembering that it means "wh0re". Interesting what that says about the psychology of the nation as a whole that they chose the word "wh0re" as not only their favourite obscenity, but one that an awful lot of people in Poland parrott out several times a minute.
More educated or better brought up people do not swear in front of their kids.
This is also true, however its the tracksuit-wearing, mulleted, church-going, bigos-eating ones ones further down the food chain whose use of it you (and the poor wee kiddies) hear.
So to answer the topic title, no this is not normal in society in Poland.
It is very much 'normal society'. Every society is comprised of different elements and the poorer and less-educated are the vast majority. Hence hearing it all the time.
I favour this approach:
Then don't. Tell them that if they want to be around you and the child, the swearing has to stop.
Having said that , two years old IS a bit young, and at that 'parrot' phase. OP just stop inviting them round for another two years.