Sonatina is okay because of the nn possibilities.
Symphony however...I just can't imagine that as a girl's name. I am a music major and for me a symphony is something to be written, performed, and analysed, not something to name a child.
I am not a fan of term names, usually, but Sonatina and
Aria would be kind of cute I think. they could be called "Sona" and "
Ari" or "
Tina" and "
Ari".