If the Alsace is on the agenda, definitely go to Strasbourg. It is MAXIMUM. CHRISTMAS. The lights, German-style market chalets, building decor -- it is really, really magical, and will be especially for your daughters. At night it is a complete wonderland.
If you know Paris already, you will also love it at Christmas -- the lights, the shop decor, etc -- but the little markets they have set up throughout the city (mostly at the bottom of the Champs Elysse near the Concorde and at the base of the Trocadero across from the Eiffel Tower) can be kind of chaotic and almost feel more like a carnival (ie, kinda low quality). It's still fun to get a cup of vin chaud and wonder thru them. Ice skating and decorations at the Hotel de Ville. Good time of year to ride the huge ferris wheel at the Concorde. My favorite thing, though -- holiday foods! The patisserie wares are SO festive, specialty food shops are bursting with fun things, and everywhere - champagne and oysters! Make sure you spend some time in some of the residential areas in the 10th, 11th, 12th (there are little playgrounds everywhere for the girls) and check out the crates of oysters stacked outside of markets, the huge champagne displays in the supermarkets. Stop for some oysters and sancerre at a brasserie in these areas, and chocolat chaud for the girls. Enjoy being outside even in the cold under the toasty sidewalk heaters.
Families get together for really, really fancy meals this time of year, and you can take part in that with a really lovely gourmet-feeling treat instead of spending big at a restaurant. Splurge on a really spendy bottle of champagne and bring some roast chicken and charcuterie from a boucherie, a bunch of radishes and high quality salted butter from the supermarket, and a baguette and lovely desserts back to your hotel or apartment.
Practical notes: If you've mostly been to Paris at other times of year, prepare for the fact it will get dark super early (if memory serves, 4:30 or 5, depending on cloud cover), but of course you know everything's lit up at night so there are still plenty of things to see walking around after dark. Plan on at least some rain. I know the temp can vary from year to year, but I found it to be much warmer in general than I'd expected, especially indoors. I packed lots of layers, my warmest coat, etc. Was completely fine walking around outside, but as soon as we went indoors anywhere would be hot and angry. Just something to consider - make sure you carry a little folding nylon shopping sac when you are out and about; I can pretty much guarantee your kids will want to shed layers immediately when you go inside!!
Have a WONDERFUL time.