I find your itinerary a bit "light". You can combine some of these activities that only take a couple hours into one day and see more. I'm not one to pack a ton into an itinerary, but for example, it only takes you a few hours to do Schilthorn....
Day 0: Arrive ZRH late morning → straight to Wengen
(Hotel Baren and "The Deck" have good food for dinner), also go to the Coop grocery before 6:30 to pick up hiking snacks for the week
Day 1: Männlichen + Alpine Playground + Lieselotte kid’s adventure hiking trail > cable car back up to Mannlichen and either cable down to Wengen or if you still have more energy, the easy family friendly walk to Kleine Scheidegg (Bergrestaurant Grindelwaldblick + great souvenir shop in KS @ Wys Sports) + optional detour up to Fallbodensee for the kneipp baths > train back to Wengen + early dinner at Allmend on the way back to Wengen
Day 2: Schilthorn + Birg Thrill walk in the AM, Murren/Allmendhubel (lunch) + easy walk down to Gimmelwald in the afternoon + dinner @ Mountain Hostel (wood fired pizza with a view)
Day 3: Lauterbrunnen Valley (Trummelbach Falls, alpaca farm) - this really only takes you a 1/2 day, so you could also go back up to Murren to do the Northface hike in the afternoon if you still have energy or just go back up to Murren to relax up in the mountains and do the easy walk from Murren to Grutschalp and then cable car back down to Lauterbrunnen from there. Or add in Schynige Platte (see note below). Hotel Alpenblick in Murren has very good Swiss homestyle food (schnitzel, etc.)
Day 4: Lake Brienz would give you something different than the rest of your itinerary but not necessarily specifically exciting for an 8 year old unless she specifically enjoys water. You can swim in Brienz at the end of the boat ride if that’s of interest. Pfingstegg would give you the adventure activities, but the views from there aren’t great, so I’d only spend a few hours doing that with extra time perhaps relaxing at the outdoor swimming pool in Wengen if it’s nice.
Day 5: Train to Zurich + Lindt Home of Chocolate
Day 6: Fly home
If you're staying in Wengen, you should do Mannlichen your first full day as a good jet lag activity. If you don't plan to go up to Mannlichen for some reason, then there is no reason to stay in Wengen and I'd stay in Murren instead. Wengen or Murren are the best base options. You can also look at the Edelweiss, Alpina, Eiger or Eiger Guesthouse in Murren.
Why in particular do you have concerns about motion sickness with Schynige Platte? I wouldn’t find it any more likely to cause motion sickness than any other mountain trains. It’s open air, slow moving and consistent, since it's a cog. It will feel less jolting than going up to Wengen at a higher speed. It’s beautiful up there and highly recommended. They play alphorns between 11-2 pm near the restaurant, there is a playground and incredible hiking trails with lake and mountain views.
Coming from NYC, I doubt you’ll really find anything that crowded. I find everywhere except the hiking trails crowded, but I live in the Tennessee countryside when we're not in Murren for the summer :) Start out when you are ready and just enjoy the journey. The most important thing to do is watch the weather (MeteoSwiss) and webcams to determine when is best to be where. For example, you can do Lauterbrunnen Valley on a cloudy day, so you should adjust your itinerary based on which days are the best to do my suggestions of Day 1 & 2 and fill in from there.
The Swiss Travel Pass is not helpful to you up in the mountains, as it doesn’t include any of the mountain destinations such as Mannlichen, Schilthorn, Allmendhubel, etc. I’d suggest the Half Fare Card + 4 day reduced Berner Oberland Pass + free family card so that your 8 year old travels 100% for free. Then for days 0, 5 & 6 you purchase half fare tickets to/from Zurich.
