My two help plan the menu, they pick from an approved list.
For snacking, we look at the cost of it vs the cost of baking, generally mine prefer to bake.
When mine get older I plan on giving them a 'budget' and letting them cook from it. I figure after a week of eating nothing but bread cause they splurged on the first meal, they will start to appriciate me more
I know some people who let the kids keep the difference of the budget. (or the szavings from coupons)