Composer: Petzold, originally attributed to J.S.
The Minuet in G major, by Baroque German composer and organist Christian Petzold, is a well-known melody popular with beginning pianists and other musicians. It is one of a pair of minuets from a harpsichord suite by Petzold, though this one is probably the more well-known of the two. Until the 1970s, it was thought to be written by Bach, as it was included in his 1725 Notebook for Anna Magdalena Bach (J.S. Bach's wife).
If you ever watched the 1995 movie Mr. Holland's Opus, you may remember a scene where Mr. Holland explained to his high school music class that this minuet was the basis for a 1965 pop song A Lover's Concerto, written by Sandy Linzer and Denny Randell and recorded by The Toys.
This is another composition that is well-suited to flower girl entrances - it's simple, even child-like in its sound (probably because it's so often learned as a beginner piano student). And it can be short or long, depending on whether the performer utilizes the repeats in the music (which is quite useful, given the unpredictable nature of flower girl entrances!).