Hi Saumik and welcome to MySwissAlps!
Thanks for your question about the Swiss Travel Pass and Swiss Family Card.
1. The Swiss Family Card is a free add-on to your Swiss Travel Pass for your children 6 to 15 years old, you can request it when you order your Swiss Travel Passes. You can purchase your Swiss Travel Passes from any of these reliable resellers: www.myswissalps.com/swisstravelpass/price.
Read more about how to use the Swiss Family Card at www.myswissalps.com/swissfamilycard/howtouse.
2. You do not need any separate museum pass, just show your Swiss Travel Pass. Here is a list of the museums where you can enter for free or for a reduced price: www.myswissalps.com/swisstravelpass/details.
3. You can buy a Swiss Travel Pass for 3, 4, 8 or 15 consecutive days. If you need it for 18 days you should make a calculation which pass or pass combinations are most suitable for you. If you do not tend to use your Swiss Travel Pass on every day in your 18days itinerary a Swiss Travel Pass Flex might be the better option for you. It grants free traveling for 3, 4, 8 or 15 days within one month (you choose for how long you want it). It also gives you the same free entrance to Museums as the Swiss Travel Pass. Check the prices here: www.myswissalps.com/swisstravelpassflex/price.
If you are unsure which pass to choose, I recommend using our calculation spreadsheet, you can download it from here: www.myswissalps.com/train/ticketspasses/practical/chooserailpass.
4. You should know that seat reservations in Switzerland are usually not obligatory except a few panoramic trains (www.myswissalps.com/train/reservations). If you wish to reserve your seats for example to the Jungfraujoch, you need to reserve 3 seats (2 adults+ 1 child).
Cable cars, cogwheel trains and funiculars are discounted by 50% with the Swiss Travel Pass. See the details here: www.myswissalps.com/swisstravelpass/validity. Your child travels for free with the Swiss Family Card. At the ticket counter of the chosen mountain transport, you'll get a ticket for your child if you show your passes. Children from 6 years old need a ticket (even if it is a free ticket) at the entrance to be able to pass the gate. (Smaller children are allowed just hide under it.)
I hope I answered all of your questions. :) Let me know if you still have some, we are happy to help.