Welcome to Chicago :-)
It completely depends on how old your boys are, where you will live and the traffic patterns of your area (ie, you don't want something 'downtown' unless you live or work downtown - or you are looking at a CRAZY commute). Have you found a place to live? Once you narrow that down you will be able to search more easily, but here are a couple initial tips -
Chicago Public School District has "Pre-school for all". Some are 1/2 day, some are full day, some are free and some are tuition based. It all depends on where you are in the city. Their website is cps.edu.
You other city-wide alternative is Chicago Park district. You can view programs for fall as of Aug 2nd and online registration is Aug 16th. The better programs can fill up WITHIN MINUTES so it's important to do some research ahead of time on what you want.
There are also a bunch of private daycares, but again you want to get one within the neighborhood where you will either live or work. "in town" can vary wildly by commute time.