At this point it is difficult to tell whether the Immigration officer will ask you to submit the photos or reject the application for being incomplete.
The best thing you can do is (and this is just my opinion), once you get the Acknowledge of Receipt email, it will have the Application number.
(Acknowledgement of receipt means that the Sydney office has received your application and it is formally in the queue for being processed).
Use that application number to link your sponsorship application to online IRCC account.
- first create an account on IRCC on your child's name (the child being sponsored aka the principal applicant).
- during the linking of application, you will be asked to enter the number of applicants included in the application. (This includes; the principal applicant, and the sponsor(you), and if there is a co-signer count him/her as well) - at minimum there will be 2 applicants (the principal applicant and sponsor).
Once linked, all the messages from IRCC will be sent to the online application. This way, if the immigration officer feels that something is missing from the application, they will send you a message on your online account and my guess is that you will be able to upload the requested documents online rather than mailing it to them.
Hope this helps and that the immigration officer won't be harsh for this minor discrepancy in your application.