i would think that for young people a dm who is "into it" would be particularly important.
snarls and growls, shouts and whispers, anything to really make the badguys and npc's really stand out (without being TOO scary).
if you can slip into storytller mode well i think it would add a special plus for kids. dm's for adults tend to rely sometimes on their players inherent knowledge that when the orc speaks it is gruff or coarse, kids may need this explicitley explained or demonstrated.
making game world scenarios analagous to real world happening will also help, remember that a childs knowledge about medieval history is liable to be rather sparse, so the fact that say, live chickens are being sold in the marketplace may have to be pointed out, or the fact that the streets are generally filthy, or that there aren't streetlights.
remember, when dealing with kids that the whole world is a new and different place, they haven't "been here before" as it were.