One of the most charming descriptions of a fancy dress ball in my collection is that of the event held at Buckingham Palace in honor of the sixth birthday of Queen Victoria's youngest son, Prince Leopold, on April 7, 1859. This was a juvenile, or children's, ball, but, as we know from descriptions of the dancing lessons given to Victoria's children, the level of dancing skill even at young ages was considerably higher than one would expect from children today. That said, it's not clear to me whether the youngest children really danced all the dances or whether that was left to the older ones, or perhaps the parents.
The description I have was printed in The Albion, A Journal of News, Politics and Literature, on April 30, 1859. The Albion was a weekly New York newspaper that covered British matters extensively and was read by expatriates. The description was probably copied directly from a London newspaper.