Mar 8, 2015 - 10:39 AM
Welcome to MySwissAlps! A Swiss Travel Pass is a good choice in your case. The trip to Mount Rigi would be free, as well as the museums you intend to visit. The children travel along for free. All details can be found here.
You can buy the pass once you arrive in Switzerland, but that takes you time and the price is lower online as you can see here (check the Discounts section too!). So unless you're not sure about which pass you want, it makes sense to buy online. Currently the best price is € 204 for a 4-day pass.
There are lockers and a left luggage office at the rail station of Brienz. I think there are lockers in the Ballenberg Museum as well. I don't know about exact prices but it will be a few francs. And there is a luggage service to transport your bags for you.
Whether the museum fits your schedule on the last day depends on what time you depart and what time you need to be in Geneva. All train schedules can be found in the timetable.