If you get the 4-day Swiss Pass for yourself, here is a really fun day you can have:
From Mürren, he'd down to Lauterbrunnen, either by the train/cable car route, or take the S hilt horn cable car down. Is Gimmelwald to Stechelberg, then Postbus to Lauterbrunnen. The latter is pretty exciting as the cable car swings out over the cliff. Then catch the train up to Wengen. This much is fully covered with the Swiss Pass.
At Wengen, walk through town to the Männlichen cable car. Your pass will give you a 50% discount, and the kids are free. Ride up and take the easy and scenic walk to Kleine Scheidegg. If it is lunchtime, stop at the mountain restaurant on your left before you get to Kleine Scheidegg for a bite to eat. The view from the deck is lovely.
From Kleine Scheidegg, take the train down to Grindelwald. This will again be free for the kids, but your pass will give you only a 25% discount. At Grindelwald, walk through town to the First (pronounced "Fearscht") gondola. Again, the kids will be free and you get a 50% discount. There are numerous adventure activities from the top of this lift: zip line, Trottibikes, mountain scooters. All are good.
To return to Mürren, take the train from the main station on the valley route back to Lauterbrunnen (via Wildsrswil and Zweilutscheinen). This will be free for the shoe way back to Mürren with your Swiss Pass, and for the kids.