I would strongly encourage you to follow Maggie's #1 suggestion at the very least. The scenery is extraordinary, and it would be an absolute shame to be in the area and to miss out on seeing it :) Furthermore, that excursion offers the chance to ride in a cogwheel train, a cable car (large, people mostly stand) and a gondola (small, seating for about 4 people), which would be great experiences for both you and your children.
Here's another idea, which would be free if you have a Swiss Travel Pass: take a ride in a train which has a children's play area on board. This might be fun for your little one. The play area is on the upper level of the first or last coach of the train.
From Interlaken Ost, it is only trains on the Basel line that have the play areas. You could just travel as far as Thun, which is about half an hour, then catch a boat across Lake Thun back to Interlaken West. The boat trip would be another fun experience, and the wharf is adjacent to Thun railway station.
For a picture of the play area on board the train, see here:
To find trains with a children's play area, use the SBB (Swiss Railways) timetable, using Interlaken Ost as your starting point, and Basel SBB as the destination. Look for the symbol "FA" in the "Comments" column, as this indicates that the train has a coach with children's play area (see attached screenshot).
For the timetable for boats on Lake Thun, see here:
(Look for the summer timetable, which covers 13 May to 15 October 2017)