If you need a simple piece for this combination I suggest you try this. A whimsical and jazz-influenced melody is passed around the instruments. There is some rhythmic unity but eonough independence to encourage the development of ensemble playing. Even the piano part can be played by a pupil - what a bonus!
Difficulty level, roughly compared to ABRSM exam grades. 0 is beginner, 9 is advanced (beyond grade 8).