Your children are 7 and 8 - are they old enough to have an allowance and for all their snacks and treats, such as ice cream, to come out of it? If you can transition to that system it could take a lot of pressure off your wife to be the regulator of what gets bought and when.
Good plan :)
Now to sell it to the wife....lol
If you haven't broached this yet with your wife, please do! My kids are that age. They used to nag all the time that they wanted ice cream at school or candy at the store or some special prepackaged snack that is SO GOOD. Some days I'd give in because I just didn't have the emotional bandwidth that day to say no One.More.Time.
So I switched to this method. They get allowances for doing chores, plus a set amount of money per month for school lunches. If they want to pack a lunch and use the lunch money to buy an ice cream, fine. If they don't want to pack a lunch and buy it instead, fine. If we're at the store and they want a giant pack of Swedish Fish, fine. If they run out of money for the month....too bad, no snacks, no purchased lunches.
It WORKS. I no longer get asked to buy those types of things. The youngest has even started spending an extra dollar to get the family-sized bag of Cheetos instead of the individual size "because you get more for your money". We write his name on the bag so the other kids don't eat it, and he can have a serving every day.
I bet your wife could modify a system like that for homeschooling, too. It's taken so much pressure off of me, and I'm always amazed that they're putting into practice the financial lessons we've been trying to model for them.
That said, I do stock the occasional ice cream/popsicles, and I surprise them sometimes with donuts or snocones. They appreciate the treats more now that they are more rare, yet they don't complain about being deprived.