I think "failling to do so " mean if you don't attend interview it will result in delay as they will reschedule your interview. your case is in Islamabad or London. if in Islamabad then email at " ISBADIMMIGRATION @ international.gc.ca"
if in London then notify at "LDNIMMIGRATION @ INTERNATIONAL.GC.CA"
In this email mention your File # and full name and also inform them about your child in same email.
About your son if your husband is Canadian citizen then apply for his Canadian Citizen ship Card from Canadian embassy in Islamabad. Try to apply for his Canadian Citizen ship card before your interview it will help you to proof that your marriage is genuine.