If there's a girl in the mix, she may very well separate all your colored metals by "pretty colors vs. icky colors", goes for matboard samples, too. (Don't ask how I know)
Can they make size foamcore scraps into piles for you?(younger ages like to sort) Get crayons (or markers if you can trust them) and give them the sheets that come between the glass to color on, draw cities, make foam core buildings, etc. If they aren't content to do crafty stuff, or you cannot contain it to a "safe zone"a feather duster or a broom as mentioned are good choices. Some kids love to clean windows,glass cases over and over and over.(my guess would be -NOT the 7 yr. old!) You may go through a bottle of cleaner, but they will stay busy, and your shop may be sparkly in the end!! Give the older kids scraps to make signs on for forts or the cities, etc. My son (7) really loves my ATG gun- I have to keep close tabs on it...
P.S. Hide the sharpie markers.