Which pass is best?

I think you’re better off with a Swiss Travel Pass because the two kids under the age of 16 will travel for free if each parent has a STP. Request a free “Family Card” at the time you buy your Swiss Travel Pass.
Why bring a rental car from Munich only to park it the whole time you’re in Lauterbrunnen? That is an unnecessary expense. You’ll also have to pay for parking, I would actually forget about bringing a car from Germany to Italy because it’s going to incur a supplemental “One-Way Dropoff Charge” of about €1,000 for picking the car up in one country and dropping it off in a different country.

The STP does not make sense for this itinerary, as they don't have that much travel listed to come close to breaking even.

Get half fare cards for the 2 adults + 16 & 18 year old and the family card for the 11 & 13 year old to travel free. This gives you 50% off everything for everyone 16+ and free for the younger 2. The Jungfrau Travel Pass would be a good fit for you if you decide to do more than what you have listed (Mannlichen, Grindelwald First, Schynige Platte, etc.) which personally I would think you'd want to do. Just going to Grindelwald is of no great excitement (unless you just want to shop and eat), the mountains above Grindelwald require additional transportation. Same with Lauterbrunnen. The town can be seen in an hour (souvenir shops). With 3 full days, I'd do

1 day - Schilthorn, Birg Thrill Walk, Murren, Northface Hike (prioritize this day for your best weather day)
1 day - Wengen, Mannlichen to Kleine Scheidegg Panorama walk + optional Grindelwald First
1 day - Lauterbrunnen Valley + Schynige Platte

I agree that bringing a car from Germany for this makes very little sense. Is this part of a longer trip and are you returning to Germany in the end?

But in case you really want to have a car, consider parking in Interlaken instead.

Yes, we will have a car as we are spending almost 2 weeks in Germany, Switzerland, and Italy and have a family of six and for pricing. It was cheaper than getting six year passes and gives us more flexibility.