We're group of four adults and one 8 year-old girl. We'll arrive Interlaken in the morning and stay 2 nights there. Here are what we're interested; Lake Thun, Harder Kulm, Jungfraujoch, Grindelwald, First, Schilthorn. We don't ski, we may have sledging, walking, and sightseeing. I'm confusing which place I should do first and later due to distance and travel time. I plan to visit Lake Thun and Harder Kulm on 1st day. But not sure I don't think I can put Jungfraujoch, Grindelwald, First, Schilthorn altogether in one day. Check the travel time from Interlaken to Grindelwald, it's 45 minutes by train. So I should put in on 1st day, what do you think? Please help me set up my itinerary.

We have Eurailpass which give 25% discount on some trains. But do we need regional pass, will it save our cost or be costly for us?

I don't know the distances from these areas, but Murren is so intoxicating, it is a bit of each of these, in my opinion! Have fun!

No, you can't go it all in one day.

First, since you are interested in the Alps, I would suggest not staying in Interlaken. Take the train south from Interlaken to much quieter Lauterbrunnen. It sits in a valley with the Eiger, Monch, and Jungfrau on one side and the Schilthorn on the other. The connections to either the Jungfraujoch or the Schilthorn are easier from there than from Interlaken.

Assuming you have clear, sunny weather do the Schilthorn on one day and the Jungfrau on the other. Weather tends to be clearest in the morning, so go early if you can. If you don't have clear sunny weather, skip them because they aren't worth it if the views are obstructed. Instead, there are lots of cloudy weather activities in the area: many easy downhill hikes that anyone can do (but an 8 year old might be pushing it), Tremmelbach Falls, biking, etc. Ask the TI and your hotel for hiking suggestions and maps and other recommendations for cloudy day activities.

The regional Berner Oberland pass is not worth it if you are only there for 2 days. Just buy point-to-point tickets. Your Eurail Pass should get you 25% off the Jungfraubahn and the Schilthorn, as well as all the lifts and funiculars.

Another vote for NOT staying in Interlaken, a charmless town that looks at the mountains in the distance, but is not in the mountains. (It also has the only Hooters restaurant I have seen outside the US, not a selling point for the town, IMO.). If you want to stay on the lake, then Brienz, Merlingen or Speiz would be better, but my strong recco is to stay in one of the mountain towns. You can take a day trip down to the Interlaken area to go to the Harder Klum, take a ferry ride, etc.

I agree with staying in the Lauterbrunnen valley. Everyone has there favourite town, mine in Wengen. However, you could not go wrong in Murren, Lauterbrunnen or Grindlewald. Staying in Wengen will save you about 30-40 minutes on the Jungfrau trip, as Wengen is the first stop (and train change) when doing this trip. Staying in Murren will save you time on the Schilthorn trip.

As for your fist day, I would let weather be your guide. If when you arrive by train into Interlaken (where you have to change to get to the mountain towns anyway) it is NOT a clear day, then check your luggage at the station and spend the day on the lake and exploring Thun, etc. If it is clear, go right up to the mountains. Weather is very changeable here in Switzerland, and if you get a clear day take advantage of it, as it more than likely will not e clear tomorrow. . . .
The Jungfrau trip is only worth the expense on a clear day, and as noted above, is best done starting as early in the morning as you can, as you have the best chance that it will stay clear. (Another good reason to stay in Wengen.)

Well .. we aren't really true nature or adventure lovers, so we pick Interlaken which is quite comfortable for us. Anyways, still open for your advices.