Royals are about as tropical as they come so make sure you
bring them in and make sure you give them plenty of light.
Mist them occasionally to keep mites off. pot them in a sand/ miracle grow soil and you will be fine.

Move inside and don't water them too much. Give them as much direct sun as possible.

Thanks for your advice. I was just looking at the historic average winter temperatures (Dec-Jan) for this area of Southern California (Camarillo) and it is 45 degrees. Would you still suggest I bring these roystonea seedlings inside the house during that winter period? Any advice will be appreciated.
Thanks.

Hi
Royals are extensively used as street trees in this area
of florida and it always gets into the low forties every winter. 35 was the low last year. I think the important thing is duration. While they obviously survive they definitely don't like it lol. I'd keep them as warm aas possible particularly while young. gary

Fl is different than much of CA in daytime temps as well.garyfla is correct on the low temps the only thing I would add is the temps in the day are just as important. As garyfla stated the duration is the key and if your night time temps average is in the 40s then that is low for royals and if they survive it will be a struggle. I am going to guess that garyfla's average area temps outdoors is 57 to 60 degrees which is a big difference.
I would bring them in for the cold nights and back to the warm sun in the day and plant them after you can't bring them in anymore. When they are trunking size they can handle cold better.

IMHO Royals are much more cold hardy that they are given credit for. There are three large one here in Houston that have survived 10 winters in the ground. I have seen them weather 25 degrees, and an ice storm in the past 3 winters

I would still suggest moving them in, especailly if you get a lot of rain. They would rot if given too much rain in the 40s. David is right, in Florida if it drops to 40 it could still easily hit 75 the next day. I'm guessing that is not the case where you are.

I live inland Ca, 19 miles off the coast,zone 9. I was able to grow roystoneas to 10 ft and lost it after that unusual cold, together with some other palms.