Two doors from Le Rostand, at 2 place Edmond Rostand, is the patisserie Dalloyau , which has some of the BEST hot chocolate in all Paris--but you probably won't find it in the guidebooks.
The sedate tea room is upstairs, overlooking the Luxembourg Gardens, the pedestrians, and traffic. Window seats are best. They bring a pitcher of hot chocolate (like at Angelina's) that the girls could share, but they may refuse to share once they taste it. Dalloyau, took over the patisserie that had been in this location for decades, Pons.
You can also buy sandwiches, pastries, macarons, quiches, etc--butter aroma included.
Luxembourg Gardens also has a for fee play ground and Marrionette Theater.
edit: spelling corrected, danger of thinking in English and unstressed vowels, while writing a French word--thanks chexbres.