Lehenga bought in March, and now our 12 year old wants trousers and a short kurta for the wedding

My niece's wedding is at the end of the month. In March we bought our daughter a lehenga for it, which she chose herself at the time. She now says she will not wear it. She wants trousers and a short kurta, 'like a normal person', so she can run around with her cousins and not spend the evening holding up a skirt. I am not attached to the lehenga. The problem is that all the girl cousins are wearing the same colour for the sangeet, my sister-in-law organised it, and the photos have been planned around it. I'm trying to work out whether this is worth a disagreement at all, and if not, how to tell the family without it becoming a discussion at the wedding itself. At what age did you start letting your children decide what they wear to family functions?