There are many interfaith families these days especially at Reform Temples and Synagogues.
Mitzvah Market has occasionally fielded questions about protocol and how to deal with family members that might not be Jewish or familiar with the Bar/Bat Mitzvah process.
We found a great Website called InterfaithFamily.com. They are an online resource for interfaith families where one partner is Jewish, the other is not and the family is exploring Jewish life. We thought the Bar/Bat Mitzvah Primer was particularly interesting. Some of the sections include:
Introduction to Bar/Bat Mitzvahs
Creating a Comfortable Bar/Bat Mitzvah for Intermarried Families
Ways the Non-Jewish Family Participates
Involving Both Sides of the Family for Mitzvah Projects
InterfaithFamily.com empowers people in interfaith relationships to engage in Jewish life and make Jewish choices, and encourages Jewish communities to welcome them. They have over 2000 articles addressing every subject of interest to people in interfaith relationships including life-cycle ceremonies.