You may feed all stages of meal worms to chickens and reptiles. My chickens even relish the dead ones, dried out and riddled with holes. They're all treats.
As for the three-drawer system, that's what I started out with, and found out after my larvae went on walk-about all over the water heater closet, then my kitchen, that it wasn't desirable. If/when that happens, you will look at a few spilled bits of bran as a walk in the park.
Look into getting stackable plastic trays with covers. I drilled 1/8" holes in the lids and on the upper portion of the sides just below the rim. This provides plenty of room for all the stages, you can separate them, cut a hole in one and glue screening over it so the eggs fall down to the tray beneath, and as your colony grows, you can always have meal worms of a size to feed out.
As for the mess, with a seven and a nine-year old, they're old enough to learn how to use a hand vacuum.