Ok. Threads like
https://www.canadavisa.com/canada-immigration-discussion-board/threads/question-about-additional-family-information-imm5406e.490808/ and
https://www.canadavisa.com/canada-immigration-discussion-board/threads/imm-5669-who-must-fill-this-out.118755/ provide the answer.
IMM5406
In the IMM5406, my confusion was with the line 'The principle applicant, their spouse or common-law partner (if applicable), and all dependent children 18 years of age or older must complete their own copy of this form'.
I assumed the 'if applicable' meant if married or the like. However, it doesn't say 'if applicable' for children.
It seems the 'if applicable' really applies to if the "Principal Applicant were coming in through an economic stream (eg FSW, CEC), as then their spouse would need to give their family details".
The IMM5406 is a generic form used for all persons attempting to immigrate to Canada who require visa to visit and who are not only traveling to Canada for tourist or business purposes for less than 6 months.
IMM5669
The same applies. as pointed out in the second thread:
"The principal applicant completes this and any dependent they have over the age of 18 that is ALSO
applying for PR to Canada. These forms are generic, used in all classes of immigration applications which is why it asks for spouses as well. You, as a the sponsor are already a Canadian citizen or PR. You do not need to fill it out because you are not applying for any immigration status to Canada for yourself."
I hope this helps collate some information.
If I am wrong about this, please let me know.