Thanksgiving dinner close to Darien
You may want to try the Roger Sherman Inn in New Caanan.
Also, there's Silvermine Tavern in Norwalk:
...and Three Bears in Westport:
Don't know how more particular CH-ers would rate the three, but they are able to accomodate larger families at T-Giving and are likely to offer traditional fare with appropriately American/Rustic charm and ambiance.