For international travel, Canada Border Services Agency recommends that the non-travelling individual with guardianship rights, whether married or not, sign the letter in front of a notary public.  So, yes, it is recommended that you sign a Travel Consent Letter to permit your wife to re-enter Canada with your children. She may or may not be asked to produce the letter at customs, but better safe than sorry.  We are happy to prepare such a letter for you (for a fee) or click here for a template.