Darrell Brooks, 39, wept as he was arraigned in court with five counts of intentional homicide. Prosecutors said further charges would be filed.
More than 60 people were injured in the attack in Waukesha. Police say Brooks deliberately targeted the crowd.
Bail was set at $5m (£3.7m) due to his extensive criminal background.
The boy killed was eight-year-old Jackson Sparks, who was hit alongside his 12-year old brother Tucker.
He had been marching with his baseball team when he was struck. According to his GoFundMe page, Jackson underwent brain surgery on Sunday.
His brother, who suffered a fractured skull, is expected by family members to recover.
During Tuesday’s court appearance, Judge Kevin Costello said the $5m bail was due to the suspect’s “extraordinary” criminal history and because he had fled previous charges in Wisconsin and Nevada.
“I have not seen anything like this in my very long career,” Judge Costello said.
Police have said 18 of the injured are children. Several victims remained in critical care in hospital.
According to a criminal complaint released on Tuesday, Brooks ignored officers’ attempts to clear away from the parade route.
One officer at the scene said in the court document that it appeared “this was an intentional act to strike and hurt as many people as possible”.
During Tuesday’s court appearance, Brooks wore an anti-suicide smock and cried as the judge informed him that each crime carries a sentence of life in prison.
Two days before the parade attack, Brooks posted bail after being charged with running over the mother of his child in a domestic dispute earlier this month.
Milwaukee County District Attorney John Chisholm said he is now reviewing Brooks’ bail of $1,000 to determine if it was set “inappropriately” low.
Mr Chisholm, a Democrat, has long advocated for lower cash bail to make it easier for criminals to be released from jail pending trial.
At the time Brooks posted bail, he was already on bail awaiting trial for allegedly firing a gun at his nephew and another person during a dispute.
Records show that he has been charged with 10 crimes since 1997.
A video has also emerged of Brooks about 20 minutes after the Waukesha parade incident asking a homeowner to come inside and use his phone.
The resident made him a sandwich and let him borrow a jacket, not knowing of the parade attack that had happened shortly beforehand.
The attack has horrified the community of 72,000 people.
Tailyr Medrano, a Waukesha resident who was attending the parade with her one-year-old son, said at a vigil for victims on Monday night that she was “feet” away from getting hit.
“I watched people going down right in front of me and people fly off the hood of the car. Just images that I will never forget,” she said.
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