Jun 25, 2017 - 11:17 AM
Welcome to MySwissAlps, Babytree!
Early December is kind of an in-between season. Quite likely there won't be snow at lower elevations yet, so you'll have to get up in the mountains for typical snow-capped mountain tops and winter scenery. That's perfectly doable with mountain trips though. Some cable cars will be shut down as it's low season, but the major ones (like Pilatus, Titlis, Rigi, Schilthorn, Jungfraujoch, Gornergrat) should be accessible.
Most of your itinerary looks fine. I wouldn't spend 3 nights in Interlaken and 1 in Grindelwald as these are close to one another. Just spend 4 nights in either town. Zermatt or St. Moritz may well have some snow in the villages already. You can follow the links I included to compare and see which appeals to you more. Zermatt would be easier to reach from Interlaken, and it's a scenic ride if you avoid the long tunnel and take the mountain route via Kandersteg and Goppenstein instead. Details are in the timetable; enter Goppenstein as a via-station for details about the route I just mentioned.
Once your itinerary is more or less completed, it's time to look into rail passes. How to pick the right one is explained here. The 8 day Swiss Travel Pass might be a good option in your case. The free Swiss Family Card allows your boys to travel along for free on all routes.