If your kid is going to leave the bike outside, drop it on the sidewalk when they run into the neighbors house, or leave it down the street when they get distracted and don't bring it home, then a Magna will be just fine. Personally, I think every kid should start out on a cheap bike. You can go to x-mart and buy a $50 bike that will last any non-biker kid a few years.
When your child starts asking you to ride a trail with her, or he's found a place to do jumps, THEN reward their interest (and hopefully their responsibility) with a good bike that is appropriate to their riding level.
I think everyone is exaggerating the analogies here (and how bad Magna, etc is). If a HotRock is a Toyota, then a Magna is not a moped, its a Kia or something. And let's not forget that to non-bikers, a bike isn't worth near the money that some people here spend. And its hard to classify a 6 year old when all you know about her is a couple sentences on the internet.
Granted, Magna is not a quality name. But I rode 3 Huffys in my childhood, each for about 5 years. I outgrew the first two, and finally snapped the frame on the 3rd. By then, though, I knew how to disassemble the whole thing, clean, and regrease. That's when I moved on to something more solid and pricier and I still have it after 10 years as a spare bike.
My point is that not all kids will wear out a Huffy/Magna in a year. Also consider the 'fiscal responsibility' when your 6 year old's $400 bike is stolen in a year.