Question: how to legally change your child's last name in illinois?
Answer: Petition for Name Change (Minor) (171-113 Rev 2/02) (2 copies required)
Name Change Decree (171-110 Rev 10/05) (3 copies required)
Notice of Motion (171-91 Rev 04/06) (3 copies required). ... 
Notice of Publication (171-244 Rev 8/11) (2 copies required).