You misunderstood and did it wrong then. It means that children older than 18 (and not citizens and PRs) have to submit forms as well. You must list your children on this form - all of them, regardless of status.The additional family information section on IMM5406 specifically says "you the principal applicant must fill out this form for yourself and on behalf of any of your family members 18 years of age or older who are not already Canadian citizens or permanent residents". We included everyone that meets that criteria but not our 2 and 3 year old Canadian citizen children. Are you saying we should add them here? We did add them as dependents on the main form after the first rejection, despite the instructions not to include lol.
(Is the wording confusing? Sure. Complain to IRCC).