Answer:
   
   Since the usual acknowledgment is by having a kiddush in the synagogue
   the Shabbat of the bar/bat mitzvah (when the child is called to the
   torah for the first time) appropriate dress is that to be worn to
   Synagogue. This means you should dress as if you were going to the
   house of someone you deeply respect. For men, suits or sportjackets
   (subdued) and slacks. Ladies should wear appropriate dresses. In
   traditional congregations, women should not wear pants, and the dress
   should be appropriately modest (covering the elbows, knees, and not
   low-cut in front).
   
   In most congregations, when you arrive, men will be provided with a
   kipah (yarmulke) to wear (of course, if you have one, you may bring
   their own). In some, women also cover their heads; such congregations
   often provide a covering.
   
   If you are unfamiliar with Jewish congregational services, follow the
   lead of others attending the services. Stand when they stand. You
   should have prayerbooks provided.
   
   You should plan to arrive on time, but no more than 1/2 hour late.