I assume you are a PR, no tickets necessary for you.
as for your son, i am not sure. i did NOT book a ticket, but my TRV went through.
i would say that the most important thing is be upfront in the TRV visa application. in my application, i very clearly declared that I am bringing my daughter to Canada on TRV and i will be immediately applying for PR for her after she comes here. It is very obvious, so no need to hide it.
type it up as a letter during your application and upload it. i am sure it wont be an issue.