Dinner for 21 in August
I grabbed my Pudlo Guide and here are places opened (according to him) in August--although I would still check. I have no idea if they can accomodate 21, you would have to check that. Where I have eaten at a place I will make a comment. So here goes.
Helene Darroze, 01 42 22 00 11--just received her third star so it would be expensive. The chef was named as chef of the year by M. Pudlo.
L'Alcazar , 01 53 10 19 99--Run by Sir Terrance Conran at 62 rue Mazarine, it might be more of what you are looking for. There is a 22 euro prix fixe with wine included for dinner.
Les Bouquinistes 01 43 25 45 94--This is a bistrot that is part of Guy Savoy's domain. I ate there a few years ago and remember it fondly although without detail. There is a prix fixe lunch for 25 or 28 euros.
Brasserie Lutetia, is in the lovely old palace hotel. The setting is very nice having been designed by Slavik. This might be just the place for your group. There are prix fixe of 34, 39 and 41 euros. 01 49 54 46 76
L Rotisserie d'en Face--01 43 26 40 98--This is an old favourite of mine, that I went to on my second trip to Paris. It is lively and fun and the rotisserie chicken with mashed potatoes is terrific, as is the rack of lamb. It is a little more than your budget, but it attracts an interesting crowd. The negatives are: sometimes very hot and I think the chairs can be uncomfortable.
And one suggestion in the 7th arr.
Le Bistrot de Breteuil, 01 45 67 07 27--There is a beautiful terrace adapted to all weather. I enjoyed a meal here last year. It is a prix fixe brasserie, at 33euros. I think it would be fun with a large family group. The service was very polite and charming.
Aren't most of those way too expensive? The sixth is not easy, especially in August. What about La Rotonde in Montparnasse? It is really good and they have a 15€ lunch menu -- 35e in the evening. It is a big brasserie at the corner of Montparnasse and Raspail, very animated, with a large terrace.
My husband and I recently attended an anniversary dinner for about 30 people at the Closerie des Lilas, Bd. de Montparnasse, in the 6th. The restaurant is open every day throughout the year. The Restaurant is a bit more pricey than 30 europs p/p I think, but they have a more casual section, which I am sure is less expensive.
re: henri cat
With such a large party, I'd definitely second a brasserie rather than a small bistro (not sure anyway, they can host and will accept such large parties).
La Rotonde is a good choice with correct prices, while La Closerie des Lilas is kind of expensive (even the more casual section) and more upscale than La Rotonde, although charming.