Swiss Travel Pass vs Oblerland Regional Pass?

12 days doesn't really fit with pass durations, so your best bet is likely to be a Half Fare Card. It costs 120 francs, and entitles you to a 50% discount on just about every railway including mountain excursions. There are some exceptions (Muottas Muragl comes to mind), but they are few and far between.

But for a definitive answer, the only way is to do the math.

Are any of the 5 kids under 16? The Swiss Pass has the advantage of providing free rides on trains and lifts for kids traveling with a parent using a Swiss Pass. The mountain lifts are mostly covered at 50%, with the notable exception of the Jungfraujoch train which is 25% above Wengen.

Unfortunately, the only way to know for sure is to do the math and list out all of your planned travel and excursions and compare prices. Because everyone wants to do different things, there's no "one size fits all" with determining which pass or combination is best; you just have to dive in to the details.

Here is an excerpt from a trip report I wrote for my family's trip to Switzerland in 2019 (3 adults, 2 kids under 16) which details the passes we purchased and why:

I have not done the math for every leg of every journey, but I am fairly sure that what we purchased was the most economical strategy. Just as important to me was convenience – I did not want to stop and purchase tickets for 5 people every time we got on a lift or a train, so I never considered just a Half Fare card. So even if we didn't max the cost savings, I am thrilled that we never had to stop for a ticket anywhere in the BO except for the Schilthorn lift, and we rode literally dozens of transportation legs.

Here's what we bought:
Husband and mom-in-law: BO passes (350CHF each for 8-day pass)
Me: Half Fare card (120CHF) + BO pass (285CHF for 8-day pass with ½ fare card discount)
Kids: free Swiss Family pass with my ½ fare card + BO pass (30CHF each)

The Half Fare card for me gave me the discount on my BO pass, discount on my Zurich to Luzern fare, plus the free family pass for the kids, so they were free Zurich to Luzern plus free up to the Schilthorn. So all of those extras totaled more than the 120CHF cost of the Half Fare card. I purchased the additional BO passes for the kids because I knew we'd be doing a good bit of travel above the upper villages, and we saved quite a bit by using the BO passes for them (plus that convenience factor). So, basically, once we were in the BO, we used the BO passes for everything; the only exception was the lift from Muerren to Schilthorn.

You will want to look carefully at the validity map for the B-O pass.

https://www.regionalpass-berneroberland.ch/assets/Uploads/Geltungsbereich-Regional-Pass-2022.pdf

Note that the highest sections of the Schilthorn and Jungfrau trips are covered at 50%, not completely. That is Schilthorn above Gimmelwald, and Jungfraujoch train above Eigergletscher. The First lift is fully covered.

How many days do you actually have in Switzerland? In can’t be 12 days, as you are traveling to Milan by way of the Bernina Express—via Varenna, or Lugano?

We will actually be in Switzerland about 9 days, with the last 3 or so being in Italy. I have added up all the train ticket prices as we travel from town to town and its about $200/person. However, I am assuming that there are a lot of incidental trips by cable car or other means one might take during the trips to and from sites that would add a lot of additional $$$. Am I correct? In our case, it seems it's a pretty close race between passes and just pay as you go.