Hi, Chollyk...
Planning a B'nai Mitzvah for your child is a once-in-a-lifetime opportunity, and you should never be afraid of making it exactly what you want it to be. In precisely the same way that a bride will include a recommendation for appropriate attire in her invitations (white tie, black tie, semi-formal, etc.), you have every right to be specific about the nature of your event (and your expectations of the attendees) in yours. So long as your choice of wording is warm, gentle and polite, no one should take any offense. Perhaps something along these lines:
Dress is semi-formal -- no jeans, shorts, flipflops, or bare shoulders (without a cover-up), please.
Please do not bring food, drinks or electronic devices into the sanctuary.
Good luck, and Mazel Tov!
Tammy
The Amazing Bottle Dancers
