Grocery shopping in Mons

Colryut for most things, Carrefour for fish, Delhaize for quality.
As you don't state if any of the kids are still in nappies or if you're a value-brand fan, cost is impossible to answer.
You can work out what you'd buy here:
https://colruyt.collectandgo.be/cogo/fr/home
or here:
https://www.delhaize.be/en-be/shop
Mons is only 30 minutes from Frence, where you ought to find larger and cheaper hypermarkets.

Best for quality is usually the local markets.

Depends what you buy... You don't say how big your family is, the composition of your family or whether you're looking for cheapest, best quality, markets, supermarkets etc. As J says, links above can guide you.