Hmmmm... My theory... Small child will be found hiding in a cupboard somewhere... Not so small child will have thought it would be a scream to pretend his kid brother had gone up in the balloon, then got too scared of all the kerfuffle to admit to it being a joke cos he'll be in trouble with a capital TR.
Either that or small child has plummeted to a nasty, untimely death. Cue hysterical outpourings of sympathy a-go-go for the "poor" parents, who left a flippin "flying saucer" shaped balloon filled with flippin helium tethered in their back yard, accessible by their idiot unsupervised sprogs.
Cynical, moi ?