The basic stuff for the kids I usually shop at CandA.
Swimwear varies a lot in price with the season.
There are also 'kinderkleiderbörse' which are childrens second hand clothing sales or shops.
I can recommend this second hand shop for kids up to around age 10-12:
https://www.kinderkleider-boerse.ch/
They resell for local families. Just watch out for their opening times - they are not open all the time.