I love this story, it reminds me of the time myself and my dad nearly burned down the house. We made a tinfoil hot air balloon with a gas camping cooker to power it. The cooker was tipped onto the lino and the melting began. It was my mums fault for leaving us alone in the house together. In general I love these homemade balloon stories, like Larry Walters across California.
Could we have told in advance it was a haox? "Few had raised the issue of whether such a balloon could even lift off with a 50-pound kid inside, and then float the way it did"
So how big was the balloon? "Authorities said the silver balloon, 20-feet long and 5-feet high, at times reached 7,000 feet above the ground while adrift. It was found more than 90 minutes later in a field near Colorado Springs."
Then it would have a diameter of 6 meters and a height of 1.5m high. Taking the balloon as a cylinder then according to here would give it a volume of 42.5 cubic meters.
Gross Lift (Helium) = Volume (cubic meters)*1.05kg
So 42.5 * 1.05=44.625 kg lift.
Now that seems just within the bounds of possibility as the kid weights about 20 kilos and the balloon itself should be light. But could the balloon have gotten to a height of 2000 meters? There are all sorts of tables for these I might work it out later
Another site gives "Helium provides about 9.8 newtons of lift per cubic meter, at sea level and room temperature. That's enough to lift 1kg". So 20 kilos would require 20 meters squared to lift it. Same result as before.
Thanks to the best program on Television we know how many balloons it takes to lift a small child.
"It would require such a large number of balloons (3,500) to lift an average four-year-old girl of 44 pounds (20 kg) just a few feet off the ground that there is no way the myth could have happened unintentionally."
Update: Wired have an article here about how people should have realised the balloon was empty. Their estimation of the carrying capacity of the balloon is similar to mine. they point out that the balloon does not move like it has a heavy weight hanging from the bottom of it.