You may want to check out a really good back by Cardogan Guides called "Take the Kids to Paris." The author goes through numerous restaurants that are great for kids.

I've got the book right here (but haven't eaten at any of these) ... in your area, she recommends:

Chez Lena et Mimile (32 Rue Tournefort) -- They have an all-you-can-eat Chocolate Mousse challenge! Not sure what that is, but you really can't go wrong.

Le Polidor (41 Rue le Monsieur Prince) -- Traditional bistro with long tables for big families. Chocolate tarts for kids (and they have the squat toilets all kids love).

Ty Breiz (52 Boulevard de Vaugirard) -- Supposedly wonderful crepes at low prices.

Hope this helps!