I'm not sure which IRCC page you are referring to. You can certainly sponsor a family member later. However it's required that they be added to the application as either an accompanying or non-accompanying family member ahead of leanding. This was the step you didn't complete since you didn't wait for confirmation from IRCC.
If there is a change in family composition / new family member, the requirement is to add them before landing. Right now it's taking 4-6 months to add a family member after sending all of the documents to IRCC. Once your child was added, IRCC would have issued new COPRs to the entire family (including the child).
What's done is done. You should be fine since you submitted the paperwork to add your child months ago. If IRCC raises any objections on the September 12th call or you don't have a conclusion on that date, then consult with an immigration lawyer.