That's an interesting moment. A long moment. Don't be afraid. There's the dessert itsel, I would say it's the « main » dessert, and around there're a lot of chocolates, small biscuits...
What is imperative in France is a « bûche » (log). You cannot have a Christmas meal without a bûche. Then, you can add an ice cream but it must be a « vacherin ». Vacherin is a kind of cake, looks like a cake but it's ice cream and meringue.
You'll find good quality « vacherin » at the boulangerie pâtisserie.
You can also have a mix of a bûche and vacherin : a "bûche glacée" is an ice cream shaped like a bûche.
You can also add a « salade de fruits ».
What to drink : champagne if you want but not « brut ». Choose a « champagne demi-sec » or « champagne rosé » less dry, it's better with sweet food. You can also drink any kind of sparkling wine : « crémant », « clairette », « blanquette ».
Never add ice to your champagne. Drink it cold but without ice.
If you're invited, you may think that French people are just impossible. I know, you need a lot of patience, because this dinner will last until midnight I guess !
People take their time, they relax, talk, enjoy. Between each course there will be some time, just to talk.
So if you're invited, be patient, but if you invite some French people, take your time too. It would be regarded as impolite to serve one dish after the other without this « talking time ». People would feel like you want them to leave, you don't want to spend time with them.
So, after the dessert, it's coffee time !
Usually, we go back to the living-room. If you invite, don't forget to bring all the chocolates. French people cannot « just » drink a coffee for Christmas, they need some chocolates although they're completly full.
If you're invited, the host will ask if you want a « digestif », if they speak in a familiar way they say « une goutte ».
Culturally speaking, men drink a digestive, women, not reallly often. Women only « faire un canard ».
Faire un canard means they put a sugar in the glass of their partner, and eat this digestif-sugar. That's it.
Be careful, a digestif is a strong alcohol (around 40-50°).