Prosecutors say Ann Migliorato, 63, of Crystal Lake broke an 11-month-old baby's ribs, when changing his diaper inside her unlicensed day care back in 2024.
Investigators say a 10-year-old girl at the daycare secretly recorded the incident on a cell phone, and then showed her mom.
Migliorato was convicted after a three-day jury trial.
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The jury was shown the cellphone video, which showed Migliorato using her body weight to push his legs above his head.
She was heard saying, "I told you, I don't play, dude."
She'll be sentenced in October.
"Children should never have to protect one another from the very adults entrusted with their care. Yet in this case, the courage and quick thinking of one young child to record what was happening helped expose the truth and secure justice for this victim. We hope this verdict sends a clear message that those who abuse children will be held accountable," McHenry County State's Attorney Randi Freese said.