I think I have a solution for you:
http://www.museumspass.com/en/More/Shop
Museum Pass - valid for over 230 museums, in Switzerland, Basel and Germany. All that for a yearly pass of 118 CHF is I think great deal.
If you are into museums, I think you will find this deal too good to be true.
Cheers!
I discovered some of my favorite Swiss museums thanks to the pass - many of which I might not have visited otherwise, but hey, I have the pass, entry is now free... why not?
A great way to get to know Switzerland.
all of them have special exhibitions and offer top quality learning and entertainment, not stuffy at all (as you might expect) but aimed at a general public and families.
http://www.uzh.ch/services/museums_en.html
depending on your kdis' age, start from the Zoological Museum, really great for even samll kids with a great program of workshops and nature films:
http://www.zm.uzh.ch/index.html
the Museum of the History of Medicine might not be for younger kids, but also extremely interesting.
There ARE nice museums around. But when you mention London, you may realize that the population of Greater London is more than the one of Switzerland ?! In case of large museums I still obey what a "guard" in the Madrid Prado in 1971 told me take two painters from this list, visit their places, generally up to four rooms, and then STOP and get OUT, as this will have been enough If you in London go to the British Museum nobody expects you to visit the entire place but to concentrate onto something you like. I remember to have spent TWO afternoons in 1972 in the Egyptian Section of that great museum.
Here a guide to the museums in Zürich
and here another one
we have never felt disappointed as a family wherever we went there was something for young and old to see and to wonder at.
in many places we like to return: a great thing are annual memberships. for instance if you enjoy art you can get an annual membership at the Kunsthaus in Zurich for just 160 CHF for a couple and then bring along the whole family for free whenever you like, exhibitions included + discount in the museum shop and the mayn guided tours and fun kids' art workshops.
this should keep you occupied for quite a few weekends
the zoo also has a convenient annual membership with a great calendar of kids' and family events, guided tours and special days all year round.
there is so much for kids here, I sometimes wish we had moved here when our children were smaller...
I read somewhere that switzerland has the highest museum/inhabitant ratio (in the world? in europe?) and whereas I'm sure london is absolutely unbeatable as to quantity and quality, you should also consider how much easier and more pleasant it is to move around by public transport here and how much less crowded attractions are, making them more enjoyable as a family with (small) children
Also look at local tourist places, they often give coupons, discounts and the like. Another possiblity, your bank or if a TCS member, often have offers from time to time as well. Once you know where to look, you will rarely ever have to pay full fare.