Q. How is it possible for someone who is not a kohen to bless a bridal couple, a Bar- or Bat-Mitzvah or a child with the priestly blessing?
A. The statutory invocation of the priestly blessing must be done by a kohen with the traditional n’si’at kappayim (“raising of the hands”).
However, by pronouncing the words of the blessing outside the statutory liturgy and without raising the hands in the priestly manner a person is clearly indicating that they do not intend to carry out the official mitzvah set out in Num. 6:23-27 or usurp the role of the kohanim.
No-one objects, for example, to parents invoking the blessing on their children on Friday nights.