Once you complete all the information in all the fields (for yourself, your partner and your child), you will click 'Next' and then you will get a screen with various documents to upload. That's where you will upload the letter of explanation and the birth certificate.
Your child is not accompanying you as your child is already Canadian. 'accompanying' does not mean whether or not your child is coming with you to Canada, it means 'is your child also applying for PR'.
There have been several topics on this, just search 'Canadian child'.