Could you clarify what you mean here in terms of not meeting the dependent children requirement? It's not clear whether you mean it was returned because children were not included in the form 5406, or because children 18 and over did not submit/sign their own copies.
Won't get into the details of the question of dependent children and the definition / test until clear, gets too complicated. But if you think they're not dependent children, on what basis? Age? Marital status?
This is exactly what the feedback from IRCC (see below). In the IMM 5406 form, we entered all the names of children on Section B - Children. The instruction states "The principal applicant, their spose or common-law partner (if applicable), and all dependent children 18 years of age or older must complete their own copy of this form." Although they are now married and now "independent", we did still enter their names on the form. However, in the IMM 008 form - Generic Application Form, we did not declare any "Dependent", could this be the issue because we added their names in IMM 5406?
IMM 5406 (for Principal Applicant): The Additional Family Information (IMM 5406) is missing or incomplete. Please provide a current, completed and signed Additional Family Information (IMM 5406) for the
Principal Applicant and all dependents aged 18 and over, whether accompanying or not.
Each individual must sign their own copy of this form by hand.
Sections A, B and C must be fully completed for this form to be considered complete (Name, Marital Status, Relationship to applicant, Date of birth, Country of birth, and Address). The street number, street name, city, postal/zip code and country are
mandatory for Canada and USA addresses. All other countries must provide the City, Province/State/Parish and Country.