G325A is for applicants aged over 14 years old.
Your kid (8 years old )doesn't need it.On the other forms like 485 you can sign your name "on behalf Of" child's name. You should provide a copy of birth certificate indicating your family relationship.
http://a257.g.akamaitech.net/7/257/2422/14mar20010800/edocket.access.gpo.gov/cfr_2003/8cfr264.2.htm