Hey, I am in a similar situation. Could you tell whether there is an option of reopening the application with the child added, without going through the pool, CRS draw and IELTS again?
Just write back to IRCC once you are ready to travel using the web form and they will direct you on what to do. Mostly I guess it will be medicals and then send child documents and passport for COPR re-issuance. I do not see nor heard from anyone that had to go through the entire process.