I believe he is asking about material to fill the legs.
Indeed, lead (or sand mixed with lead) was formerly a popular material for weighting and damping, and particularly for rack legs, until people became more conscious of the potential health risks. Even if you are not ingesting it or directly breathing it in, thermal changes can result in the release of toxic fumes (lead oxide). Unless you are convinced there will be a reliably tight seal, lead is simply not a great material to be around, especially for children.
Sound Anchors uses sand in the legs of their steel racks. I don't know for sure, but I would guess they use blasting sand, but not sure which grit. A mixture of grit sizes will be denser than a single grit size but, I wouldn’t over-think it. Medium grit would probably be suitable.
