The girl was swimming with a camp group from New Berlin.
"She was in approximately four feet of water, pretty much in the middle of the pool. She was a foot under the surface when she was noticed," Captain Phil Horter with the Brookfield Police Department said.
The girl was rescued by lifeguards who began to perform CPR before rushing her to Children's Hospital, where she is in critical condition.
FOX6 News is told other off-duty medical personnel who happened to be at the pool also jumped in to help provide medical attention until an ambulance arrived.
"(The lifeguards) are certainly trained to a high level, but having to react to something like this and then go right back to work can certainly be difficult," Horter said.
It was a hot day that turned into a traumatic experience for both swimmers and lifeguards at Wirth Park Aquatic Center.
"We just came out here just looking to have a little fun and go to the water park, and it was kind of emotional. Some people were crying," one swimmer said.
The Wirth Park Pool was shut down for the remainder of the day on Tuesday, following the incident. The pool was scheduled for holiday hours Wednesday (the Fourth of July), but may remain closed.