Louvre

When I went there were at least three lines. One for museum passes, one for pre-purchased tickets and one for people who still needed to buy tickets. You don't have to buy them well in advance. Don't buy tickets if you are thinking of getting a museum pass. Having something in hand will keep you out of the longest, slowest line.

If you have pre-purchased tickets and show up at least 30 minutes early, you will get in quickly and find yourself in a surprisingly empty museum - no crowds at Mona Lisa if you go straight there.

Children 17 and under are free to most museums in Paris. We went to the Louvre last week and walked through security at the Carrousel Entrance with no line at all and had maybe a dozen people ahead of us for several machines to buy tickets. Very fast. If you can print tickets at home that might be convenient, but in my experience the Louvre is not a ticket issue. The Musee d'Orsay on the other hand is a nightmare if you don't have tickets or a Museum Pass to use the special door for pass/ticket holders.

Great advice. Do I have to buy a museum pass for the kids?
Really only think we will go to orangerie. Orsay and louvre.

Kids 17 and under are free; buying a pass would be wasted money; they just walk in with you. A 12 year old is not going to be asked for age ID but a 17 year old might be.

You can also buy Louvre tickets at the tabac shop in the Pyramide underground mall. It’s between the Louvre entrance and the metro entrance, you’ll see some signage for it as well. No lines at the tabac stand, I do that every time.

Depending on how long you are there and how many museums you plan to see, the Paris Museum Pass may save you money. It's good for 2, 4, or 6 consecutive days (compare prices with what you plan to see). It doesn't let you skip the security line, of course. I bought my on my first day at a tabac across from Sainte Chapelle. See the website for details. http://en.parismuseumpass.com/

I discovered in April that the weather in Paris is very changeable, so I stopped relying on the forecasts. But you can purchase your Louvre tickets the day of as described by a previous poster if you want to wait for a rainy day (just remember the Louvre is closed on Tuesdays).

Have a great trip!