Toni,
While all of Switzerland is scenic, I'd probably skip Bern, Avenches and Murten and focus on Lucerne and Locarno. That will provide a nice blend of German and Italian areas.
The route I'd probably use is Lauterbrunnen > Lucerne > Locarno, although that would depend to some extent on where you're going after that.
A few thoughts......
- the trip from Lauterbrunnen to Lucerne is only about 2H:25M, so very easy.
- while in Lucerne, you could easily do a day trip to Bern if you want to see the Bear Park or whatever. It's only about an hour each way.
- your children might enjoy the fantastic Museum of Transport in Lucerne. You could easily spend the better part of a day there.
- you might also like to visit the famous Lion of Lucerne as it has an interesting history, and was referred to by Mark Twain as "the most mournful and moving piece of stone in the world."
- while in Lucerne you could also take a trip to Mt. Pilatus
For the trip to Locarno, one route you could consider is to travel by train to Domodossola (about 3 hours or so) which is on the Italian border. From there take the scenic Centovalli Railway to Locarno (that trip is about 2 hours as I recall). Locarno is worth a few days and while there you could......
- visit the impressive Madonna del Sasso Franciscan Monastery.
- take the cable car up to Cardada & Cimetta, which provides incredible views of the lake and the city. You could so some light hiking up there and have lunch at one of the restaurants. There's a restaurant right next to the top cable car station that has a great patio, and you can watch the Paragliders sailing by while dining.
- stroll along the waterfront
- explore the many stores and restaurants surrounding Piazza Grande.
Depending on where you're going after that, you could return to Stresa by boat and then take the train from there to your next destination. You should easily be able to fill in seven days in those two places.