Not only did balloonbrat Falcon Heene casually admit they "did it for the show," but this video of the UFO taking off demonstrates that there couldn't be any kids inside. It also demonstrates that the father is a world-class a-hole.
Clearly, that balloon didn't have a kid inside. It's just flying up way too fast and uncontrollably. If the kid would have been on the bottom part—weighing the balloon at its center—the balloon would have taken off in a straighter vertical line, and never tilted in the way it did.
Later in the video you can also see the bottom part on its own, after the authorities got the balloon. It looks like it was completely sealed from the start, so there's no way the kid could have got in there at any point after the construction of the balloon.
Look at the guy getting mad and shouting at his wife, as she fails to hold onto the balloon tether. As Adam Frucci just cleverly remarked a minute ago: "I knew Wifeswap was a sleazy show, but this is some next level shit."