Versailles

Versailles Express from Eiffel Tower - http://www.versaillesexpress.com/en/c/schedules-and-rates - adult 44 eur round-trip

metro+RER Translinien Paris Nord - Versailles  RIVE GAUCHE Château de Versailles - 3.45 eur

http://www.babble.com/babble-voices/what-to-do-in-paris-with-kids-my-childrens-top-10-list/
The actual Palace of Versailles is a pain in the butt. You'll wait in line a while -- we waited an hour -- and once you get inside it's so crowded you can hardly see anything. The real treat is the gardens. Way in the back you'll find the Petit Trianon, and behind that you'll find the Queen's Hamlet, or Hameau de la Reine. My guidebook didn't even mention the Hamlet! It's like a little movie set of a rustic village from the late 1700s, featuring thatched cottages, a pond filled to the brim with fish, gardens full of flowers and vegetables, and a farm with cows, goats, sheep, donkeys, chickens and even rabbits. The Hamlet was far and away the biggest hit of our trip to Paris with the kids -- an absolute delight. They imagined the Queen romping around with her friends, pretending to be one of the "unwashed masses," which is exactly what happened here.
http://en.chateauversailles.fr/discover-the-estate/le-domaine-de-marie-antoinette/the-queen-hamlet/the-queens-hamlet 

Kogu Versailles Passport 18 eur
The Palace 15 eur
Trianon Palaces and Marie-Antoinette's Estate 10 eur

http://mylittlenomads.com/paris-for-kids 
Open 9am to 6:30pm from April 1 to October 31. 9am to 5:30 from November 1 to March 31. Closed Monday and public holidays.
An awesome combination of superb palace and stunning gardens. Makes a great day trip from Paris which is 30 minutes away by train (and then a 10 minute walk from Versailles Rive Gauche train station to the palace). Tuesday and Sunday are the busiest days – and the website has a updated chart on what days will be busy and quiet. Gardens are stroller friendly but strollers can’t go into the palace rooms. Be prepared for lots of walking. Bike rentals available from both near the train station and inside the palace complex. Row boat rental at the garden’s Grand Canal.