brooke,
Not to worry, the group here will be able to help you get this sorted.
To begin with, my suggestion would be to stay in Lauterbrunnen as it's the transportation hub and therefore convenient for travel to both sides of the valley. Some complain that it's in the valley and therefore doesn't have the spectacular mountain views that Mürren offers, but it's plenty scenic for me. While Gimmelwald is also somewhat scenic, it doesn't have much happening in the evenings (might be boring for the kids). Mürren will probably have more "kid friendly" activities.
As I always advise for those planning to visit Switzerland, bring money as it's an expensive place to travel. Are you also planning to visit the Schilthorn?
This website might help in deciding which rail passes might be most appropriate.....
The Half Fare Card is a good solution for many situations, but you may find that the Berner Oberland or Jungfrau Regional Passes are also a good option......
The issue of which Swiss rail pass to buy is something that many here have difficulty with, and I've even resorted to formatting an Excel spreadsheet to try and make some sense out of it. I'm not sure what the ticket details will be for the children, but hopefully one of the others can help with that part.
The best idea for clothing is to pack with the concept of layering in mind. That will be especially useful on the mountain tops. With the trip to the Jungfraujoch, it's a really good idea to check the weather before venturing up there, as if the weather is bad you won't be able to see much.
One other important caution to mention is that the Jungfraujoch is at an elevation of almost 12,000 feet / 3575 metres. You'll have to watch the children in case they develop any altitude-related issues.