Paris
Left Bank

The area of the Left Bank that encompasses the Latin Quarter and St-Germain de Pres is the site of ancient Paris. The Romans founded a town here. In the Middle Ages, colleges and abbeys popped up on every corner, scholars came from all over Europe to study in Paris. The illustrious University of Paris, founded in the 12th century, continues to this day, and includes the Sorbonne. A nice way for kids to begin exploring the Left Bank is to walk over from Notre Dame on the Pont au Double,

National Museum of the Middle Ages (Musee National du Moyen Age) - This museum is a double whammy. It combines a lovely medieval residence and exquisite goodies from the Middle Ages, with nicely preserved ruins of ancient Roman baths.
"Biggie" in the museum is the incomparable Lady and the Unicorn tapestries. The six tapestries are displayed in a room where you can walk up very close to them, close enough to see the amazing details of flowers and trees, and all kinds of animals (one tapestry alone has 11 rabbits, one fox, two dogs, one lion cub, and the unicorn).
On the ground floor are dazzling stained glass panels from Sainte Chapelle, sculptures from the outside of Notre Dame, and the Roman baths.
Upstairs, along with the Lady and Unicorn tapestries are galleries with crosses of solid gold, silver reliquaries, Visigoth crowns, gold and enamel jewelry, armor and shields.
Outside the museum, in the gardens, inspired by medieval designs, are benches, and a small playground with climbing structures and bouncy things, perfect for toddlers.
Jardin des Plantes -
Dodo carousel (Dodo Manege) - Charming carousel with extinct (or endangered animals), kids can ride on a Dodo, Triceratops, Atlas lion, Tasmanian wolf, Glyptodon (giant armadillo), Sivatherium (like an okapi), horned tortoise, panda, elephant, or gorilla.
Natural History Museum (Museum d'Histoire Naturelle) - The gallery has a spectacular display of animals lined up Noah's ark style, but not all exhibits have labels in English.
Jardin des Plantes Zoo (Menagerie) - This is a small zoo with a variety of animals - birds, reptiles, flamingos, orangutans, ostrich, yak, deer, camel etc., and is most fun for toddlers and younger kids.
Luxembourg Gardens (Jardin du Luxembourg) - If you've seen enough Paris museums for the day and the kids need a change of pace, head for the Luxembourg Gardens, the largest green space on the Left Bank, and one of the most stellar public parks. In summer, there are tons of green chairs where you can flop down (while the kids run around), and lots of benches and places to picnic.
At the Grand Bassin (large shallow pond), rent small sail boats (boats have colorful sails in red white and blue or pirate motifs) and launch them on the pond.
Look for the small scale Statue of Liberty, a model by the original sculptor.
The large play areas with colorful climbing structures are good for kids of all ages. Little ones with enjoy the carousel and pony rides.
There are also classic Punch and Judy puppet shows regularly (next to the carousel). Check the newspaper for times.
Ice cream (4, rue Vavin) - Just a few steps away from the park, stop for an ice cream at Amorino, absolutely delicious ice cream in a wide variety of flavors.
Le Bon Marche - It's fun shopping in this historic department store in St-Germain. The upper level has a great selection of lovely kids clothes for boys and girls (beautiful princess dresses), and toys.
Don't miss the Epicerie, one of the most beautiful supermarkets in Paris. Here you can stock up on a boggling collection of chocolates or everything you need for a picnic - freshly made sandwiches, cheese, bread, fruits, cold drinks, fabulous pastries and cookies.
And, there's a small park next to Le Bon Marche, in Square Boucicaut, right next to the Sevres Babylone metro stop, with benches and a little kid's playground.
Catacombs visit (1, place Denfert-Rochereau) - In the 1780's, cemeteries in Paris ran out of space, so bones were re-buried and neatly stacked in tunnels from the old limestone quarries under the city. The sight of thousands of bones and skulls can be scary or fascinating (or both). If it does appeal (we happened to visit the catacombs on Halloween, and my boys loved it), it is a memorable walk through underground tunnels under the streets of Paris, and your kids will get incredible bragging rights.
Tip: It's about a 2 kilometer walk, down a series of spiral stairs and then some distance walking through the tunnels, and all those bones, so this is a visit with older kids. You enter the catacombs at place Denfert-Rochereau and exit near avenue du General Lelerc.