I'm not sure if children's books are the best choice for learners. They only seem easy, but as a matter of fact, they are full of vocabulary you are hardly likely to use in a conversation (unless you enjoy talking about dragons or castles).
I don't recommend Polish coursebooks either - I have seen just one but it was disasterous!
How about primary school books that teach kids to read? They are based on simple words and easy to follow even if you study by yourself.
Good luck :)
There are many Polish book stores in Chicago.They have everything you need in Polish. Many, children books. School books, adult books. All in Polish.