For second-hand I can recommend -
max & moritz - http://www.max-und-moritz.ch/

Also a searchable database for 'Kinderkleiderbörse"
http://www.kindex.ch/index.php5

C&A is my favourite for reasonable price and range.

You could also order stuff online - cheap and basic quality - www.bonprix.ch
the advantage is that they you can do it by 'invoice' so you order first pay later...