I’m concerned that a floor length Bistro dress wouldn’t give her enough room to walk/play. You couldn’t simply lengthen it, you would have to alter the shape of the skirt by slashing and spreading it. I agree that the attache dress would be more appropriate for concert wear. It’s also extremely easy to fit and sew because of the princess seams. You might also consider the cinema dress.
My mom and I used to make lots of concert dresses for me for orchestra too. Back in the early 90s it was impossible to find teenaged sized clothes in all black! To this day, my favorite thing I ever sewed was a floor length black skirt for concerts. I lengthened it myself – which then seemed like such creative altering! lol. I always felt like a grown up pro in that skirt. I wish I still had it.