Xiao Baitu and others, please help me on my family's 485

My application is based on EB-1b. Now I'm preparing my family's 485.
Could you tell me who can sign my hu*****and's 485 form? If I sign my son's (4 years old) 485, should I sign his name or my name? Do I need a G-28 for my son?

Thank you very much.