Many of internationals go to the Cafe Imperial Sheraton Warsaw Hotel & Towers, ul. Prusa 2, tel. 022 450 61 80. It's about 150zł/person.
I'll give them a call but the (three) reports I've heard are that it's not up to much foodwise. I must say that the sunday brunch there is particularly unimpressive. But thanks for the tip.
McDonald's
I don't think I can imagine a worse meal than the MrWigilia, a carpburger?!!!
Bristol hotel's "Malinowa" restaurant,
That one I do know about: it's 275zl per person. And as it's going to be me, my girlfriend and her daughter, would put a bit more of a hole in my pocket than I want to be there (especially as I'll be dropping 500zl+ on Christmas day brunch).
You can switch to english and make a search for reustarants in Warsaw opened on 24th. All opened ones will have a green token next to them on the list.
That's a good link, hadn't seen it before. Unfortunately it looks a bit bleak for Christmas and I do wonder if all the info is correct, they list one place as being open even though I know it isn't (because a friend of mine owes it and told me yesterday that they are not open).
Europejski hotel's restaurant,
Hasn't the Europejski been shut for a couple of years?