I usually go to my local Coop around 17:00 on Saturday, to try and get the meat / goods that are 50% off.
One tip though is do your grocery shopping in Lidl and Aldi. Way cheaper than Migros and Coop.
It won't be a deal breaker..
However, you should also be eligible to receive family allowance, which is 300 chf, per child per month in Geneva.