Well I was reading the thread, and found that I might have to provide one Self Declaration. Wasn't sure what context needs to be in there.
I found one reference to a self-declaration in this thread, and nothing else on the internet. (It's gibberish BTW, because a statutory declaration can only be made by the author/signatory - so 'self' means nothing.)
Anyway: a statutory declaration is basically a type of affidavit - a statement you make in writing. But if you don't know what it is supposed to contain, doesn't mean much. ('Statutory' may or may not mean it should be witnessed by someone. Instructions matter)
Presumably here it says something like "I swear I have no contact with or power over that kid whose other parent has custody and I hereby recognise that I am forswearing all rights to sponsor that kid in future."
[signed, self]
But unless you find out from IRCC what they want it to contain, won't do much good to make one.