The following info on Paris relates to my last visit there in September,
For travel on the Metro system, I found the carnet (10 tickets for 48 F) to be the best deal. It saved 40 % over the single ticket price of 8 F. The Carte d'Orange Coupon Vert (pass for one week) was 75 F. For your situation (3 people) 3 weekly passes cost 225 F, which is the equivalent of your family of 3 taking approximately 16 Metro rides together. Walking is a great way to see Paris so if your family is able to do some walking, I doubt you will take 16 Metro rides.
The Carte Musees et Monuments cost 160 F for three-day pass. I found it to be extremely worthwhile as it allowed you to peek into some smaller museums at no charge that you would not ordinarily pay to see. It was delightful to drop into some small museums you were walking by and spend 15 minutes inside, longer if things interested you. My museum attention span is no longer that 90 minutes, so I took one of the daily English speaking Louvre tours (90 minutes, which hit the highlights) which I would highly recommend as the Louvre is so huge and overwhelming. I love Impressionist art so my favorites were Orsay and Marmottan, and of course Rodin Musee had tremendous sculpture. Consider the following single admission prices (Major musee have no charge for children under 18): Louvre 45 F, Orsay 39 F, Sainte-Chapelle 32 F, Napoleon's Tomb 37 F, Arc de Triomphe 35 F, Sewer Tour 32 F, Picasso Musee 30 F, Rodin Musee 28 F, etc. Decide what you might want to see and do the math.
Note: Monday the Orsay and Rodin Musee are closed as well as Versailles which makes the Louvre more crowded. Tuesday The Louvre, L'Orangerie and Marmottan and most national museums are closed which makes Orsay and Versailles jammed. Wednesday and Thursday all museums are open.