We just did four days in Brienz. Train and bus to 'Beatusholen' (caves, has a good play ground for kids and an awesome waterfall, and a few other features)...
Rothornbahn was awesome, but also very scary for those who have a problem with heights (and not that kid-safe at the top - it's a long way down without any fencing)...
Ballenberg was greatly improved with kids/grandma by paying 40chf for the horse and cart ride, and not aiming to see everything - we saw enough in the 5-6 hours, and at the centre of the property there is a nice outdoor cafe with a playground for kids...
On our final day we went up Haslital from Meiringen and that was pretty nice too.
Next weekend we have a booking at a chalet up from Murren. It's a 'hostel'/ Pension...we're going to do the Schilthorn and then to Murren and then come home via Thun and go to the Beatusholen again (to take Dad and our guest) and then Interlaken and home to Zurich...
With kids I can actually recommend the youth hostels - they don't mind noisy kids so much, usually have a decent play area indoors and outdoors, and have cheaper options for breakfast, lunch, and dinner...
I can tell you that I enquired at the youth hostel in Grindelwald and they had a 4 bed room, you could also try the YHA Bönigen. They also had a 4 or 6 bed room available.
For family bookings my technique is to email the hostel directly, their website doesn't always show the available options (they are sometimes willing to 'convert' a shared dorm to private family use if they have one available).
more info:
http://www.youthhostel.ch/en
I'd also recommend Brienz as a central location, and they have a youth hostel too...but I don't know about their availability. We stayed at a private b&B - called 'Trauffer' - it was just an old house where the rooms are basic (but comfortable) and breakfast was the usual swiss bread, cheese, coffee or tea and jam...but our host was very friendly to the kids and part of the property has an indoor heated pool and she loaned us the key...it was just about the cheapest I could get for our family - around 100chf for a double and 130chf for a triple room...
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