Welcome Bea!
There are many different restaurants in town from casual to fine dining. Check out their websites and menu descriptions on the Mammoth Lakes Tourism site:
http://www.visitmammoth.com/things-to-do/restaurants/Many restaurants will offer a special Christmas dinner menu and set price for adults and children and take advance reservations. Once you find a place that appeals to you give them a call and see if they're putting on a special and if they're taking reservations then make them soon! Town is at it's very busiest from Christmas until New Year's so you don't want to be left out in the cold waiting in line on Christmas Day for a decent place to dine.
Enjoy getting to know Mammoth Lakes and Mammoth Mountain...it's a wonderful place!

(But I'm partial since I've been a resident here for 30 yrs.

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