I am pregnant with our fifth and the oldest is five years! So I certainly understand how it is. First of all, don't buy high gal low nutrient items. Sometimes my girls like a bowl of cereal for a snack, a slice of toast, they like to make their own PB and J sandwitches. Keep crackers on hand (non salted saltines are good) for crackers and PB, lots of fruit and the little bagged carrots are handy.
I never bought white bread so they were used to eating whole wheat. A boiled egg can be a nice snack and is common in Asia. Nuts add protein and I like to keep a snack mix on hand of nuts, cheerios, raisins, mini marshmallows and pretzels. They like cheese too and I just cube up a block of cheese.
I make casseroles and the such on the weekends and freeze for times when I don't feel like I can cook. I also use the crockpot to cook large amounts of chicken or beef at once. I then freeze them in 1 meal portions and use them for tacos, chicken salad, soups, ect.
A great standby is mac n cheese. Cook the pasta and while doing so put some milk, cheese and butter in the microwave or melt on the stove. If it's too runny when you put it in the pasta, sprinkle in some corn starch or flour to thicken a bit or let it cool and the cheese will firm up.
S., mom to 1 in heaven, 4 on earth, and 1 in the oven!