Renaissance BJ (the one on Xiaoyun Lu)--I think they've got a Thanksgiving turkey dinner for RMB 238 + 15% service, 12 and under 1/2 price. I think it's buffet style.
Since we'll have folks from Haidian coming, something that's within reasonable walking distance of a Line 10 station (or Line 10 + short cab ride) would be most useful.
Most of the other hotel Th'giving dinners are much more expensive (Grand Millenium, Ritz, etc). So I think Chef Too and Ren BJ Chaoyang are probably the best we're going to find if you want the traditional dinner.
Other option is to not get fixated on roast turkey and pumpkin pie, and just find a completely unrelated restaurant to have our feast (though it would be nice to skip Chinese this time around). My two mao.