Well, let's see if I got another access tomorrow. With the first email, they haved asked my details and my child details. But with the second email (portal access), there are only my details...I think you need to provid your kids information too, then they can add under your case, or create another one for your kid.
Or at least reply the email to ask what you need to next.
Why from the beginning to the end, it can't be easy with IRCC lol.