We booked Delta out of Chicago back in 2007. We checked in online prior to leaving home for the bus stop after getting our kids up at 4am to make the bus to the airport. We were dropped off at the Delta terminal with our kids and all our luggage, only to find out our flight was cancelled!
When I found that out, I demanded to be put on another flight and they put me on the next flight out with United (which luckily was nonstop as opposed to ours that had one stop). We were lucky to get on that flight and all sit somewhat together.
The only problem is that to get from the Delta terminal to the United terminal is a major PIA with two kids and a bunch of luggage, two booster seats for the kids. No offers by Delta to help us get to the United Terminal (we had to walk outside the airport through a construction zone for a ways to get there...what a pain!)