Based on my own experience, and on conversations at Drupalcon San Francisco, I think on-site child care may be a well received option at Drupalcon Chicago. It does require some space, and organization. I could help with the organizing, and raising money for the childcare and space. This does not mean that it would not cost parents who use the service in addition to their registration fees, but hourly childcare is much cheaper than a nanny full-time plus travel expenses.
Drupal is a work-from-home friendly software, and I have a feeling more people would come who can't otherwise because of being single parents, or where both parents working in Drupal. The company that I received a quote from was http://www.kiddiecorp.com/. They did OSCon 2002, and many other conferences.
The cost varies on the number of children, and how much we charge the parents for it, and is in addition to the space. Total for 15 kids (including parent's cost), is less than the cost of a gold Copenhagen sponsorship.
What I'm needing right now is some gauge of interest both on the side of the organizers and parents. I don't want to run survey's with parents without it being a possibility if there is not enough space at the venue, or if there are other concerns. As I said I can help with organizing the childcare side of things in anyway needed.