There are some nice Dvorak pieces at that level: I forget which numbers, but two of the
Four romantic pieces would be manageable, officially I think one has been grade 5 and one of the harder ones has been grade 8. There's also his
Sonatine.
Dancla's
Airs Varies are fun, possibly a bit tricky in places for grade 6.
I also like the Schubert Sonatinas - a few easier movements have been set for grade 5, they probably go up to grade 7/8.
In terms of Kreisler, if you don't want to buy a whole collection she could start with Aucassin and Nicolette, Tempo di Minuetto, and maybe Schon Rosmarin. But there are harder pieces like Praeludium and allegro she'll probably want later too.
Mazas studies are nice too - op. 36.
Lots of the above is available free at IMSLP.org by the way - although you may want an edition with good fingerings and bowings, I guess it depends whether it's just for playing around or studying with her teacher.
There are also some great Sheila Nelson collections which span about grade 5-7: I had "The Baroque Violinist" but there were Classical and Romantic volumes too which looked equally interesting.
And I echo the Bach double.

Linked to that, there are also Telemann canonic sonatas - there's one of them in the Sheila Nelson Baroque book.