It doesn't have to be related to their current piano curriculum, it can be very different. I have found that some of my most masculine piano students like the most sentimental tunes. Some of my most feminine students love songs that are loud, fast, and booming. Think about what motivated you at their age and level and try the same music.
Here are some pieces that motivated me to learn...
1. age 9 - "Toccata" by Jack Butler
2. age 11 - "Tell Me the Old, Old Story" arr. by Rudy Atwood
3. age 12 - "My Tribute" arr. by Dino Kartsonokis
4. age 13 - "How Rich I Am!" arr. by Harold DeCou
5. age 15 - "Rhapsody in Blue" by George Gershwin
6. age 15 - "Clair de Lune", "First Arabesque" - Claude Debussey
7. age 16 - Congregational hymns, choir specials - for church
8. age 17 - "Revolutionary Etude" - by F. Chopin
9. age 18 - "My Heavenly Father Watches Over Me" - just to play for a soloist
I learned many other pieces but these were above and beyond what my teacher required of me. I wanted to learn these pieces desperately just to play them for myself.
Think about each of your students, pray for them, go through some of your old music and see what name comes to mind with each piece. Watch their eyes light up when you hand them a new piece to practice!