I think you can just cover the shelves below with some fireproof tarp, as the really dangerous hot sparks from a dremel tool will fly away for no more than 3 feet in my experience. In some gentle projects I've been using a plastic bottle cut to the proper shape and attached to the Dremel thread to collect most of the dust right away.

Use large fiberglass reinforced cutting wheels if you need to cut steel for more than an inch long around especially in an awkward spot. Another option - very thin carbide wheels - they are cutting really fast at high speeds, however they are most brittle ones and can not go deep enough sometimes.

Also I would rather spend some time additionally securing stands by anchoring them to the ceiling, placing something under, or tie/clamp it down to a long enough splint to hold pieces together... as cutting with the Dremel may take quite a while, so your son may loose attention/caution. He could hold a vacuum cleaner hose collecting the produced dust instead.