Another potential issue - it appears her son was a minor at the time of the application and had a baby. If he was not married and was not in a common-law relationship up until landing, then:
1. the mom was supposed to fill out a Financial Evaluation form: http://www.cic.gc.ca/english/pdf/kits/forms/IMM1283E.PDF because she had a dependent son, who himself had a dependent child(her grand child).
2. she was supposed to declare the grand child
3. she didn't do this(based on OP's comment), and it's a material misrepresentation since it led to an error in the administration of the Immigration Act. The officer could not assess whether or not she had the minimum income necessary for her family size. This is a big deal.
If he was married or in a common-law relationship at anytime up until landing:
1. he was no longer considered a dependent and should have been removed from his mom's application
2. he would have had to find his own way to get to Canada, fill out forms and declare his baby and wife/common-law partner
As it stands, no matter which scenario, the baby can not be sponsored ever. The mom and her son's PR may be in jeopardy over this misrepresentation. Perhaps this information would spark some interest on her part to try to resolve this issue.
Lastly, officers ask before completing the landing procedures whether family composition, etc has changed. This was another opportunity to declare the child but he didn't. Cutting corners does not help in the long run. To ensure all avenues are explored, best to seek legal advice - some offer free consultations.