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.