I didn’t mean to imply a 6 year old WOULDN’T want to do it but that they COULDN’T do it.
If that balloon had enough lift to carry the kid then it would be too challenging for the kid to keep it tethered/balanced while he climbed in and then release the tethers.
But someone pointed out his brother may have untethered it for him and that is plausible. But I am highly suspect of the original story that the boy did it all by himself.