Dirty shoes during a performance are incredibly distracting. Reiterate the importance of rehearsal shoes and performance shoes to your studio families for the applicable styles of dance (ballet, jazz, lyrical, contemporary).
REHEARSAL SHOES: Shoes that are worn in studio on a regular basis for practice, rehearsal, class, etc.
PERFORMANCE SHOES: Shoes that are broken-in AND stage ready for appearance.
Also, make sure there are no functionality issues with the shoes- no holes, no loose soles or insteps, and guarantee the security of laces and elastic.