The sponsor does not complete 5406. This is outlined in the complete guide / instructions
Additional Family Information (IMM 5406)
View form IMM 5406 (PDF, 570KB)
Who needs to fill out this form?
This form must be filled out and signed by:
- The principal applicant,
- All dependent children aged 18 or over (whether coming with you to Canada or not).
- Note: if your dependent child is the principal applicant on a sponsorship application, you should complete this form on their behalf regardless of their age.
If you forgot the birth dates of those two individuals, I can only assume they would send it back to be corrected, but I'm not sure how strict IRCC is regarding those fields. I assume the applicant that had mentioned getting their application back regarding missing the USA address is important as this family form outlines potential people that could be sponsored in the future (based on a reading I found).
If you do receive the application back for any corrections, unfortunately the clock starts over completely when you send it back. So your starting point would be the new received date by IRCC and you would need to wait for AOR.
I don't know what stage of the process is consider safe for not having your application returned for missing information, or if there even is one.