I still occassionally play D&D and still enjoy my fantasy metal quite a bit, I can speak from experience. It CANNOT be harmful to kids reading and playing such stuff, with or without family support and Christian teachings. There quite simply is nothing pro-satanic or pro-pagan in them (if you've listened to Iron Maiden then you know whenever there's someone treating with demons and such in their songs then something very bad is about to happen to that person).
No the problem is people that see something that isn't there and embarass their own faith by warning about non-existent boogiemen.
In truth the only people I know in those ranks that didn't turn out OK were already messed up long before they started playing D&D or listening to fantasy metal. Kids that are already messed up will tend to turn into messed up adults with or without D&D, Iron Maiden or Harry Potter.
Having been on the convention committee of a sf/f/horror convention for the past decade and change I can tell you the people who lurk there are often in any demographic you're looking for. Yes there are some that are hostile to Christianity, there are also some that are devout Christians. About the only demographic you aren't going to find at a convention are people that don't like sf/f or horror, which makes sense. I have no problem exposing kids to these people, they're just people like anybody else, maybe a little wierder, but at least they're avid readers which puts them in what I consider to be a superior minority in our current society. I wouldn't expose kids to the late night crowd, but the late night crowd tends to be rather... adult so that's kind of a no brainer.