Proper dress length for Bridesmaid, evening wedding

My daughter is getting married and having an evening wedding which will take place at 6:30 p.m. She is very unpretentious and does not care for long formal dresses for her bridesmaids. She will be wearing a long wedding dress but it will be more streamlined, with a small train. She has found a dress for her bridesmaids that is knee length. I’ve tried to explain to her that an evening wedding is more formal which would indicate a longer dress for the bridesmaids but she really seems set on the shorter dress. If she chooses to use the dress will it be considered wrong wedding etiquette?

Rebecca Black, Etiquette By Rebecca

Dear Alsmom,

Not really, as long as the wedding is semi-formal. Street length could be worn if it seems that female guests would wear appropriate cocktail dresses to the wedding. Of course the attendants’ dresses would be of very high quality, so their’s would be a bit more formal than the guests’ dresses. But, the bridal party’s attire should be in the same formality. This is very important.

Best wishes,