I can't speak for Southern California, but I can speak to North Florida. I definitely use a quilt (or more) in the winter. In the summer, since I sleep with a fan blowing directly on my bed, I even need one then too. (I know I could turn off the fan but then the room would get too stuffy.)
One option you have, if you think a regular quilt will be too warm for them, is to use flannel for the lining instead of batting.
I say give them the quilt. They will in time acclimate to Southern California weather and they will begin to feel the chilly nights.