I usually monitor the cereal, with no full time kids in the house it's mostly just the three adults and we eat different cereals though we will eat each others as well, but I tend to keep up with them as I do the shopping and if it's been open too long I toss it. However we tend to only have one or two boxes at a time, usually cheerios and honey nut chex, and we go through it at a reasonable rate.
If the twins are around they either eat what we have (they like the two above), or I buy them something they like that I or their grandmother will eat as well, like frosted flakes, apple jacks, captain crunch, rice crispies, (you know breakfast candy LOL) so that we don't have a half eaten box of cereal hanging around.