One of may favorite Christmas’s pieces for piano and voice is “Come, Thou Dear Redeemer” by Cesar Franck. The key I’ve seen it in is “D major”, but I’m sure you can get it in a more friendly flute key.
The melody is so beautiful and the accompaniment blend together nicely. I think you would love it if you’ve never heard it before. Good luck on your program.
I’ll add that to my list! :-)