The Musée d'Orsay is located along the Seine river, in a former railway station which was renovated in 1986. It hosts collections from the 19th century and the beginning of the 20th century, in particular some very famous impressionnist paintings from Monet, Manet, Renoir, Degas...
It's one of Paris biggest and most famous museums and therefore one of the must-see places when you visit the city for the first time (and the second, and third, and...).
The museum organizes very nice workshops and visits for children from 5 years old, the program can be found here.
To reach the Musée d'Orsay from Ivry, simply take the RER C in Ivry-sur-Seine toward Paris and get off at the Musée d'Orsay station, et voilà !

