They are such different experiences -- I really think you have to do both.
In order to "understand" or get a sense of Monet's work, you have to go see the paintings at the Orsay. If she hasn't seen those (or his paintings at the Met or the National Gallery), she may well not understand why you are going to see this place called Giverny at all.
By contrast, I would go to the Orangerie after having gone to his gardens and house. You will see those paintings differently after having been in place, and knowing he made them towards the very end of his life as a gift to the nation. As others have said, it's a very different experience sitting and seeing the (few, huge, set aside in a special place built just done them ) water lily paintings than it is wandering around a museum looking at various works.
If you do end up going to the Orsay, I highly recommend going straight through on the main floor all the way to the back, and taking the escalators up to the top floor, and visiting the Impressionist galleries first.. The Orsay can be overwhelming, and if you try to see everything, you may run out of juice before seeing those works. In order to maximize your energy and be able to see them at your best, see them first.
What a wonderful thing to do for your granddaughter. I hope you have a wonderful trip.