The importance of good sight reading skills in youth orchestra auditions cannot be underestimated. With all things being equal, the better sight reader will outrank the inferior one.
Forty variations, op. 3 by Sevcik has a variety of exercises in various keys that will be useful to anybody preparing for youth orchestra auditions. Another option is using Wohlfahrt’s 60 Studies for the Violin, Op. 45. Like the Sevcik, Wohlfahrt’s studies give violinists experience in playing different keys. Another way to practice different key signatures is to use different hymnals. I have noticed that the Lutheran, Baptist and Methodist hymnals have many flats in some of their hymns. To prepare for some of the above hymns, a Catholic hymnal might serve as good preparation as there seems to be fewer flats in these hymns.
For the advanced violinist, “Orchestral Excerpts From the Symphonic Repertoire for Violin” by Josef Gingold comes in three volumes. In addition to practicing various keys as the previous books/hymnals afford, these excerpts provide the aspiring violinist practice with reading high notes.