One bullet struck 3-year-old Deonta Howard just behind his right ear and exited from his face. He has had surgery and is recovering but is expected to need plastic surgery.
Two teenagers were also among the wounded.
The assailants then got back in the car and fled.
Police gave a slightly different account, saying Young was armed with the assault rifle and Champ with another weapon. In another difference, McCarthy said detectives believed there were no individual intended targets and that the assailants hit the park because it was part of territory controlled by the Gangster Disciples.
Neither the police nor the state's attorney's office could immediately reconcile the differing accounts.
The two guns have not been recovered, prosecutors said.
Authorities say Young has more than a dozen previous arrests and have described Champ as a felon and a documented gang member.
Champ, who was convicted in July 2012 of unlawful use of a weapon by a felon, was previously sentenced to Cook County Jail's boot camp.
McCarthy and Mayor Rahm Emanuel cited the charges and Champ's record to reiterate their demands for tougher gun laws in Illinois.
"We need a three-year minimum penalty for illegally carrying a gun on our streets," Emanuel said. "One of the shooters should have been behind bars rather than in Cornell Park on Thursday night."
McCarthy said the investigation continues and more people could be charged.
Police had recorded 306 murders and 1,402 shootings this year in Chicago as of Sept. 15, down 20 and 22 percent, respectively, from the same period in 2012. Police say overall violent crime is down about 15 percent, according to figures released Monday.