How do I buy museum pass in advance

I would buy it when you arrive at the airport. Each airport in Paris has more than one (in each terminal) a tourist information center where you can talk to a live person.

I wouldn't. I buy mine at the first venue I visit that is included in the pass. You can pick somewhere smaller (like the Pantheon) and just pop in and purchase. Or buy one at the ticket counter at the venue, but then you will have to wait in the line.

You don't need a pass for the child. She enters for free without a ticket at most of the covered locations. See also this recent report: https://community.ricksteves.com/travel-forum/france/paris-museum-pass-my-recent-experience.

Get it at the airport and then you have it when you need it but you don't need to pay extra to have it shipped or worse yet get some sort of coupon and have to go somewhere to pick it up. The 13 year old doesn't need one -- just walks in with you most places.

Buy it at the venue. Dont do it online, you will get scammed so hard. Just buy it at the first venue.

It's not a "scam" to order it online from the Paris Museums Pass website but shipping is very expensive and, as others have said above, it's just not necessary to do so. Buy it when you get to Paris, as we did, at any of the Points of Sale listed:

http://en.parismuseumpass.com/rub-t-points-of-sale-3.htm?cat=3

I just wouldn't buy it at one of the most-visited attractions, such as the Louvre. You'll want to have it already in hand for avoiding standing in a ticket line there. Also be sure not to date it until the day you're ready to use it.

http://en.parismuseumpass.com/rub-t-how-to-use-it-19.htm

The FAQ page might be useful if you have other questions:

http://en.parismuseumpass.com/rub-faq-16.htm

I got mine at the tabac (newsstand) across from Sainte Chapelle.

Picked ours up at the Conciergerie (just steps from Notre Dame and Sainte-Chapelle). No line for the Pass, plus we enjoyed visiting the museum section which we would have otherwise overlooked. (Takes less than an hour; very interesting, including Marie Antoinette's dwelling prior to execution.)