I've heard that after you do your Master in Switzerland you get 6 months working permit so you can look for job. There is no such thing with postdoc I guess? No any ways to stay longer and search for job?

Thanx for your advises in advance!

I should mention also that it is my husband who is doing postdoc and I've got visa B - regroupement familiar (family permit based on his permit)
Is there any chance that I if I find a job I got my permit prolongated?

No, no 6 month grace period for a post doc that I know of. Maybe if you get a job you could change your permit from a dependent to an independent one and he would then become the dependent on your permit, but you should talk to your cantonal migration office to make sure if that's possible or not. You'd also have to earn enough to support both of you of course.

And what is minimum limit required for 2 people with a child? And in case I got a PhD and student visa I still need to fulfil this salary requirements?

You'd have to check with the migration office to be sure as I think they vary from canton to canton. The general daily figure to work to is CHF100 per person. Normally I'd say you could get away with calculating on CHF50 per day for the second person, but if there's a child too then the CHF100 might be a better figure. Depends how old the child is, how cheaply you could live and what the canton will accept. Talk to them and see what they say.