Change ITIN to SS #

You mentioned that the children were born in South Africa.  But if the American father satisfied the US residency requirements, his children would acquire American citizenship at birth.  Did they apply for an ITIN because the residency requirements were not meet?

Were these SSNs issued only in 2020?  If so, leave the 2019 return as is.  Just make a note on your file to use these on the 2020 return.

If these were issued in 2019 and you must correct them, the best way to do that is to call the IRS, which is resuming operation in phases.  However, especially if no 1099/1098s were issued for 2019 under those SSNs, there's really no urgency to get that updated.

You didn't tell us the citizenship of the father, but it seems fairly obvious that he's not a U.S. citizen.  I would file a paper 1040, mark it "SUPERSEDING RETURN" at the top margin, and document that you mailed it by July 15.  

These instructions from IRS probably should have been rewritten after the law was changed to require an SSN for the CTC, but the part about getting the SSN before the due date of the return should still be correct (someone will let us know if it isn't).  

Note: If you don’t have a TIN by the due date of your return (including extensions), you may not claim the CTC/ACTC on either your original or an amended return. However, if you apply for an ITIN on or before the due date of your return (including extensions) and the IRS issues you an ITIN as a result of the application, the IRS will consider your ITIN as issued on or before the due date of your return. Also, you may not claim the credit on either your original or an amended return for a child who doesn't have an SSN by the due date of your return (including extensions), even if that child later gets an SSN.

https://www.irs.gov/faqs/childcare-credit-other-credits/child-tax-credit/child-tax-credit-4