Another 2cents from me. We actually tried to book Le Cellier for this September (and we will keep trying) but as an alternate we have the San Angel Inn booked and we are very happy about this.
San Angel really has excellent food and a great setting.
If you are into wine and want a romantic special dinner for just the adults (if your kids can stay with the grandparents) try Bistro De Paris. It is expensive (not on the dining plan) but what a wonderful meal and atmosphere. They do request you do not come in shorts and T-shirt but we did see people dressed that way.
I kidded my husband that when we go back to celebrate our 20th anniversary without the kids we'll eat at some of these places. For now we are eating at where we know the kids can find something. Thanks for all the advice and information everyone. We leave in 14 hours!! I'm so excited I can barely stand it!