Hello!
Does anyone know the name of the restaurant that is in the Marias where Rue de St. Croix de la Bretonnerie Ts into Rue de Vieille du Temple - there's a bakery, and, immediately to the left is a small place with a restaurant in back where the locals eat.
the Marais is the most entirely tourist occupied part of Paris; there is no restaurant 'where the locals eat'. They will all be full of tourists.
The restaurant is owned by Xavier Denamur who has been featured in the New York Times and owns Les Philisophes and a number of other places. From what I have seen, his places are frequented by locals and tourists. You are not finding some secret locals-only spot when at one of his places but the food can range from decent to good.
Plus, it’s tiny. I doubt they could comfortably accommodate the size of your party.
You want local restaurants, look at the 9th, 10th and 11th. But a lot of local places are small. I just don’t know how a group your size will be received.