If a Permanent Resident fails to disclose a family member at any time prior to or at landing, he or she will be barred from sponsorship under the family class for the undisclosed individuals. This seems to happen regularly and for a variety of reasons. Sometimes there’s an issue with the custody of a child; sometimes they get bad advice; sometimes they make an assumption that they’ll sponsor their husband/wife/child after they’ve settled in Canada. R117(9)(d):
(9) A foreign national shall not be considered a member of the family class by virtue of their relationship to a sponsor if
(d) subject to subsection (10), the sponsor previously made an application for permanent residence and became a permanent resident and, at the time of that application, the foreign national was a non-accompanying family member of the sponsor and was not examined
There is a possibility that a report alleging inadmissibility will be sought against the sponsor on the grounds of misrepresentation. The PR does have the ability to appeal against any resultant removal order to the IAD.
In any event, if the sponsor wants to bring his or her (excluded) family members to Canada, they need to submit a “H&C” or humanitarian and compassionate application relying on s.25.1(1) of the IRPA.
25.1 (1) The Minister may, on the Minister’s own initiative, examine the circumstances concerning a foreign national who is inadmissible — other than under section 34, 35 or 37 — or who does not meet the requirements of this Act and may grant the foreign national permanent resident status or an exemption from any applicable criteria or obligations of this Act if the Minister is of the opinion that it is justified by humanitarian and compassionate considerations relating to the foreign national, taking into account the best interests of a child directly affected.
In regards to H&C applications, a number of factors are to be considered (and raised in your submissions) as per the CIC website. There are a number of cases that should also inform the submissions, such as David v. Canada (Minister of Citizenship and Immigration) 2007 FC 546 (CanLII):
 The exclusion mentioned at paragraph 117(9)(d) of the Regulations is a very harsh one, but does not prevent a sponsor from invoking H & C grounds considerations. Indeed, the very reason why the Court of Appeal in De Guzman found that this provision is compliant with the international instruments to which Canada is signatory is that section 25 of the Act enables the Act to be administered in a compliant manner (De Guzman, at paragraphs 102 to 109).