Police say Fernando's father, Fernando Margarito Sr., took the boy without permission during an authorized visit on Thursday. He is not the child's custodial parent.
Margarito also is believed to have taken Susana, who is the boy's aunt, without her parents' permission.
Officials say Margarito called the boy's mother and said he and the children were going to Mexico. She called police. Early Friday morning, Margarito's vehicle crossed into Mexico. It isn't clear if the children were inside.
Fernando, 2, was wearing a gray shirt with white/green stripes and Converse shoes. Susan, 14, was wearing a white shirt with pink stripes, a blue skirt and pink sandals.
Anyone with information should call Glendale Heights police at 630-260-6070.
