The victims were rescued at North Avenue Beach Friday night.
One was rushed to the hospital.
Dangerous conditions in the lake may be to blame. A rip current advisory is in effect through Saturday.
Lifeguards attempted to get people out of the water at closing time, closing North Avenue Beach after lifeguards pulled the 12-year-old girl and several of her family members out of the water.
"It just seemed like it got deep and then the current, it just seemed like it was pulling, so I had three of them right, and then as the current came, it just started pulling us down," said Timothy Hollins, uncle.
Witnesses say lifeguards began CPR until paramedics arrived, and they transferred Ayanna Patterson to Lurie Children's Hospital in critical condition. She was apparently dragged underwater by rip currents.
"If you don't know how to swim, you have to be careful with waves," said Jackie Daniels.
"Most people don't know how serious this is. I didn't. This is one of my first times actually being on the beach during a rip current," said Melvin Sewell.
Earlier, divers pulled the body of a man in his 60s out of the water.
Rip currents also claimed the lives of two boys near Indiana Dunes in separate incidents earlier this summer.
Ayanna's family is praying her survives.
"I pray that she will be OK because she's young and she was so happy," said Hollins.
A short time after lifeguards pulled her out of the water, they had to go in after another child. That person is reportedly OK and needed no medical attention.
Rip currents are common in low spots, breaks and sand bars near structures like piers. Look for a channel of choppier, churning water. Look for a discolored area of water that might be rushing away from shore. If you are at the beach, obey the warnings. If you are caught in one of these rip currents, try to remain calm. Try to swim with others, not alone.