I thrashed a walmart Mongoose MTB. The Aluminum A-50. Its a great bike, I gave it to my girlfriend as it was to small for me but I came off BMX bikes so I liked the smaller feel on the MTB at first. The bike stood up to every thing I threw at it of road and I only tacoed a rim when I was messing around doing some flatland frestyle with it.
As for kids bikes, what a far way they have NOT come since I rode frestyle. They have gotten crazy heavey and crazy hard to do what it is most kids do with them. Ride. The frame profiles have dropped and the kids think its cool to put the seat as low as it will go, they ride around with their knees practicly hitting the handel bars and their feet almost hitting them in the but. With as heavy as they are, I cant see walmart bike being far off a $300 red line for even a moderate compeditor. The frames are not going to break under a 95 Lb kid, and there are no components on them like brakes or shifters, ect that are worth the extra $$$$. 3 piece cranks are great for a rigid crank, but not worth the extra $ they charge for the name brands. I dont know, maybe parents dont care about spending $300 on a bike for a kid, my parents would have flipped.
In all my years of bike abuse I have only ever broken 2 bike frames. A dimond back Ultra light BMX racing frame, and a Treck Carbon Road bike frame.
I guese what Im trying to say is, its mostley about the rider, not the bike. If the bike is sized and speced right for the application, It dont matter how much it cost or if it came out of a wal-mart or a fancy little bike shop owned by some racer/rider.