I haven't used Megabus personally but my daughter has ridden on Megabus multiple times. Unfortunately she has not taken it into Chicago so I cannot give you any specifics about your travel destination.
My daughter has always been very happy with her travels on Megabus but she was traveling as a college student so no family or child experience here. She generally used it between St. Louis and Kansas City. Megabus does not have terminals, they have a bus stop with a sign. Everything is done electronically and you cannot purchase tickets from the driver.
We did have occassional issues with the bus running late. During one or two trips the driver showed a movie on the bus. There were no scheduled stops but the driver always made one stop whenever he/she felt like it. Sometimes the bus would stop for a food break just outside the departure city and sometimes the break would be shortly before the arrival city so it was not possible to plan if she should eat before leaving or not.
The buses were always clean and she felt safe riding them. Beyond that I can't help much but since you hadn't received any other replies yet I wanted to give you the little info I knew.