At
Disneyland, an outfit is generally considered a costume if it closely resembles a full character ensemble or if it could be mistaken for what a Cast Member would wear.
Your snowflake dresses with capes sound more festive, as long as they don’t replicate a specific character (like Elsa from Frozen), you should be fine. Themed or dressy outfits that aren’t exact replicas of a character are typically allowed. The cape is a nice touch and should be okay as long as it doesn’t create the illusion of being a costume or possibly cause a tripping hazard.
If you’re still not sure, I would check with Guest Services ahead of time for clarification. Either way, it sounds like you’ll look amazing for your dining experience and photos with Santa. And, the weather looks great. Enjoy your trip to the Grand Californian Hotel and Fantasmic!