Since they are on maintanance level carbs or above, cereal occasionaly for breakfast won't hurt. Get a whole grain cereal though without alot of sugars. Read the lables as some of the cereals marketed as healthy have more sugars then cocoa puffs. I do the Multi Grain cheerios since they seem to have alot of nutrients.

Get LaTortillas and make them wraps instead of samwiches. My son loves a peanut butter wrap. I also do iron kids wheat bread. It has higher fiber then most of the breads, lower carbs, and is iron and calcium fortified.

For the one who can handle dairy, string cheese can be a good snack. There is also soy cheese that comes in string cheese. You can also make them a "pizza" like snack where you dice up some chicken breasts, then cook them in some lower carb marinara sauce (Ragu light has no added sugars and is cheaper then LC brands) along with some cheese.

Get the unsweetened ketchup and all natural peanut butter. Chances are they won't know the difference unless told.

For my 4 year old I'll occasionaly let him have pria bars. They don't have sugar alcohols and are lower sugar then most protien bars. While it isn't the perfect snack, it works when we sleep in and have to get him to preschool fast, or other times there isn't any way to get a meal into him.

Since they are young and don't have weight issues, don't make food an issue. Just give them some protien in one form or another and some veggies at each meal and try to talk them into eating (I'm amazed you can get them to eat veggies). Have fruits be a dessert or snack instead of carby sugary treats, but don't totally ban all sweets or they may go over board when they get some. Just relax and be confident you are giving them healthy eating habits.