Many Orthodox Jews do sit shiva for a child who marries out
but it is not, to my understanding, mandatory that they do so.
As far as I am concerned, any parent who would consider their living child dead,
violates far more Torah Laws than did the child in his/her choice of a mate.
Hashem gave that child both life and free will. Ostensibly, that free will was
to lead that child-person on a Torah path. However, free will is just that and
allows for walking off the path and either returning to it or not at some later
date. Wishing for or considering "deathing" anyone is, by my standards, an
absolute sin against the creator.
Interfaith marriage is a very common practice these days which I attribute to a
lack of adherence on the part of the parents. Telling a child to go to Hebrew
School, synagogue, marry Jewish is, of course, all well and good. However, all
those instructions become meaningless when the parents don't practice what they
preach...3 days of shul a year and a nice family get-together called a seder
with tons of food and zero spirituality hardly sets an example for children to
Furthermore, with the divorce rate reaching closer and closer to the marriage
rate, many parents have little, if any, problem "dating" a non-Jew. The message
to the child, then, becomes, it's no big deal to violate our religious laws.
That being the case, it is totally hypocritical for a parent to object to any
violation of Jewish law.