I agree completely that the TGV direct from Zurich is your best bet, especially travelling with a baby. The direct trains get in at Gare de Lyon, which is reasonably central and has lots of nearby hotels. I don't know if there's a Marriot there but there is a good Ibis just down the street, and the underpriced Palym just across.

Staying near the station is an advantage when it's time to come home.

We find that the best way to get around in Paris is on foot. I suggest getting a baby back-pack if you haven't already got one. In fact we got ours there near Gare de Lyon at Go Sports.

There's lots more info on the Wikitravel page for the 12th arrondissement.