See 6.4 of the FSW manual
"All family members, accompanying or not, are required to be examined unless an officer decides
otherwise. Normally, any inadmissible family member would render the principal applicant
inadmissible as well [A42; R23]. There are, however, two exceptions to this rule. The first is the
separated spouse of the applicant. The second is when the applicant or an accompanying family
member does not have legal custody of their dependent child or when they are not empowered to
act on behalf of that child, by virtue of a court order or written agreement or by operation of law."
click here to read more
http://www.cic.gc.ca/english/resources/manuals/op/op06-eng.pdf