Petite Litanies de Jesus - I first came across this exquisite little gem for piano probably more than 20 years ago when a friend and fellow pianist asked me to play for her wedding. She had chosen this piece for her flower girl's entrance. I had never heard it before, but found it was actually included in the Royal Conservatory Grade 8 Piano syllabus. I loved it so much that I have taught it to some of my senior-level piano students since then, and of course included it in my wedding music repertoire.
It's certainly more complex than, say, Mozart's Minuet in G, but it's only a single page, and can be cut down to the first couple of phrases (with maybe a cutaway to the end if more time is needed). That's what I have found is sufficient for a flower girl to walk in to. There is a sort of old world European cathedral sound to it, even a hint of monastic choir in some of the music. Quite lovely, and not a common choice for wedding music.
While there are arrangements for orchestral instruments (see the sax and brass recordings below), for a wedding ceremony I prefer the original piano version. It would also be beautiful done by harp.
Sheet music (piano solo) - Royal Conservatory book containing
this composition, among others of various composers
Sheet music (piano solo) - "L'almanach aux images" book (complete collection of Grovlez piano pieces including this one)