Here is my opinion and what I will do written below
I will add my minor daughter in 2 forms
1. hib_AINP0009 - Page#8, Section D. Family Members
2. imm008egen - Page#2, Details of Family Members (I don't see anywhere in this section written that the dependent of age 18 or older)
imm0008_1e is background check form and that will not be required for the dependents below age 18.
hope this will help!!
This is CIC form. IMM0008egen - Application for Permanent Residence.
But on AINP form, they also need the dependent child to sign. How did you achieve that?