This months MMWR contains a report on an outbreak of shiga-toxin producing E. coli O157 at a petting zoo in Florida in 2007.
Unfortunately, this is nothing new. The "editorial note" states that between 1991-2005, the CDC received reports of 32 outbreaks of E. coli O157 that "were associated with animals in public settings." Beginning in 2001, the CDC and then the National Association of State Public Health Veterinarians (NASPHV) has provided a compendium of recommendations to avoid outbreaks like this. The CDC states, however, that venues in some of these outbreaks were not in compliance with those guidelines.
Apparantly, those failures continue. In the 2007 petting zoo outbreak, the CDC found that attendees of the petting zoo "were not instructed in appropriate handwashing technique, and the staff member was stationed too far from the handwashing facilities to observe handwashing behavior."
E. coli O157:H7 is generally found in ruminants, including cows, sheep, and goats. So, if your child pets one of these animals, make sure his or her hands are washed thoroughly with soap and running water immediately after contact.