I would introduce another factor too.
Work out how much money you spend on food for meals and how much on snacks (ideal plus replacements).
Then sit down with your dc and talk them through the budgeting of your weekly foodshop - including things like if you have a big expensive roast on Sunday then you stretch it to last for Monday too. And then have a cheaper thing like macaroni and cauliflower cheese the next day to balance things out over the week, you can't have steak every night no matter how much you love it as it's just too expensive. Etc etc etc
Then say you are not going to buy any more snacks for them; they are going to get a budget of 50p/day or whatever you would like them to be spending and let them buy their own.
It will make them appreciate that there is not a bottomless pit of money for snacks, will get them down to the shop or Market to bargain hunt and therefore want less to eat as not at home bored. Might even get some co-operation between dc if they join up to buy a loaf to share. Provide a fridge/non-fridge box each and see what comes out... Be reasonable, especially to start, so say that they can have a week's money in advance. And if they realised say that it was cheaper to make a batch of cookies or bowls of porridge then be prepared to sub them extra initially so they can buy all the ingredients to get going (eg big bag if flour works out cheaper than smallso you could sub them 2 weeks and they would have enough to make cookies for 3 weeks instead of 1 - making sure you didn't pay them again until their subbed money had run out).
Also would help them to learn things like the Market can be great for fruit, especially just before clOsing time if they were not too fussy about whether they have apples, oranges etc but best value on the day.
Or, Consider doing a Baking thing with them - filling simple things like flapjacks or muffins, they get a set of ingredients to make something and it has to last a week. Each child bakes a batch (or pair of children)- again might help reduce the boredom/comfort eating thing.
Finally I would sit down with them all and say you are having this problem, it involves them and you haven't been able to solve it yet so what do they suggest? Might make them think a bit harder about what they are doing!
Good luck!!