Jardin des Plantes

http://www.davidlebovitz.com/2009/04/ten-great-things-to-do-with-kids/
Along the Seine in the 5th arrondissement, the Jardin des Plantes is a grand botanical garden in the middle of the city and a favorite spot for strolling. This is a good place to let the kids run and sniff flowers. There’s also a ménagerie (zoo) with some cool monkeys and an old school natural history museum with huge, dusty dinosaur bones and not a trace of the frenetic edutainment that goes on in so many museums these days. Here it’s just bones–tons and tons of them–all crowded into a majestic hall. You feel like you’ve stepped back into the 19th century. The kids are awestruck.

http://about-france.com/paris-free.htm

• Jardin des Plantes botanical gardens (opposite Gare d'Austerlitz). One of the world's oldest and largest botanical gardens, open to the public since 1640! There is paid entry for the "menagerie" section, specialised in the preservation of endangered species. However the small pandas can be clearly observed for free from the botanical gardens.