here's the report about Zeughauskeller as promised!
so we arrived half an hour earlier because it was raining and the shops had already closed (you know the problem in zurich... bad timing on our part, we should have booked at 6 pm instead of 7 pm so as not to have to wait in the rain).
the restaurant was full with people waiting to be seating at both entrances. this was a bit unpleasant, especially with our little boy, who behaved wonderfully though. we queued up and asked nicely if we could be seated a bit earlier - understably, no chance, but they assured us we would have our table as booked at 7pm. we were a bit sceptical seeing how full the place was, but when we went back we indeed were assigned our table more or less on time.
it turned out they have tickets for reservation that they put on the table so you can sit down and eat but knowing already that you will have to leave at a certain time.
we didn't have a buggy but a "child rucksack" (?) for my nephew - quite big so we had to leave it at the door, which we also could understand and were fine with.
once seated, our first encounter with the rather rustic waitress was not very encouraging: she replied very loud "You are not alone here!" just because we had looked and smiled at her trying to catch her attention. hubby was already boiling and ready to leave, but luckily the rest of the italian family didn't understand, so we smiled away and gave the sporty lady a second chance. it proved to be a good strategy because
when she came she showed a certain efficiency and even smiled and joked, even offering a few words in italian. so my conclusion is that to be a waiter there in that apparent chaos you to be a real dragon. I can sympathise, after all.
to make our order as efficient as possible we had of course chosen all our plates beforehand. my brother in law who speaks nothing but italian was sitting in front of an enormous poster with their "deer + rösti" speciality and when the waitress took our orders he just pointed to the poster, so nothing could go wrong

the rustic lady appreciated our efforts to make things easy for her and even more the fact that our little 2-year-old had fallen asleep after a loooong day on his little pedalfree bike around zurich... by the way the long seats on the side make a perfect bed for a tired child

so, after having chosen the most adequate place for him, we could actually have gone to any other restaurant, after all
as for general child-friendliness, at the table near us a little girl was crawling around the table and sometimes screaming or being loud but this seemd to cause no trouble to guests and staff.
but we are very happy we went there, especially my brother in law loved it and found it extremely "tipico" and I can really recommend it if you have family visiting. as some of you were saying the military and other swiss paraphernalia (representing the history of the building, where the city arms were kept, if I understand correctly) gives a nice local touch without being too kitsch, quite a full immersion in swissness and a nice change to the mostly "chic" and minimalistic modern restaurants in the city centre.
be prepared to hear a lot of english and other languages being spoken though, it was full of expats and tourists (but certainly not only).
we will definitely
avoid going there again on a saturday, it was simply too full - although I must say we were pleasantly surprised at how efficient the service was notwithstanding the crowd.
on the whole a pleasant experience to be repeated, so thanks again for the recommendation!