Thanks for the shoutout, 3.
I live in a well-to-do area and you would not believe the dresses some girls wear to B'nai Mitzvot... Ironically, the Christian girls usually come more tastefully and appropriately dressed than many of the Jewish girls do. I think it's because their mothers actually bother to ask what proper attire is supposed to be.
For a May ceremony, I think that a pretty dress in a pastel color or a floral design is appropriate. If Duckie has dramatic coloring, then think about a dramatic color for the evening outfit. Please steer away from black - I usually wear very little but black but it's not for pre-teens and not for a bat m since black is a funeral color for Jews. Even I don't wear black to b'nai mitzvot. It doesn't have to be tea length or have long sleeves but it should at least cover the knees and the shoulders. Technically, the elbows are supposed to be covered as well, but for a non-Jewish girl attending a Conservative ceremony, covering the shoulders is sufficient as long as they are fully covered. The cap sleeve, if Duckie likes that style, is suitable. Try to avoid anything that is too form-fitting, transparent or clingy. I prefer dresses to skirts and blouses because they look more party-like and you avoid the issue of the blouse riding up (I have been known to safety pin the shirt to the inside of the skirt).
For the parties that daughter went to, I bought her dresses from several sources. However, we got the best, prettiest and most reasonably priced dresses at a thrift/consignment shop in a wealthy neighborhood. I even bought her prom dress in a place like that - she looked like a Disney princess for $25 in what had to be somebody else's expensive custom made gown. For stores, I recommend Macy's (you can shop online), Lord and Taylor, Estelle's Dressy Dresses (it's a local place down here but may have a website; it caters to a large Jewish clientele).
To me, the major thing is that a 12 or 13 year old girl should look like a 12 or 13 year old girl.
Is the evening party at the temple or at an outside venue? If it's at the temple, her dress should be a little more conservative - shoulders should be covered. Also, keep in mind that a lot of the activity at a typical party like this involves dancing and games - many parties give out socks for dancing but you might want to give her a pair just in case. I also don't recommend very high heels, flats are best for dancing and standing.
Also, if she's never gone to a bat mitzvah before, look up some youtube videos of people doing the hora. It's an Israeli folk dance that is always done at Jewish parties - the entire guest list is supposed to participate. It's fairly simple (even 3 left legged me can do it, but I learned before kindergarten) and since I seem to recall you saying that Duckie dances, so she should pick it right up.
If you have any questions, feel free to ask away. I just left the Bar M circuit last year... having been on it pretty much since 1994.
