They did. Based on the initial dimensions of the balloon that authorities had it was calculated that it was possible to carry about 100 lbs on the outside edge. But, for one thing, the dimensions of the balloon were actually a bit smaller than what Heene reported to the authorities. For another thing, I don't think they could have called B.S. at that point. You have to go on worst case scenario even if they're calculations showed that the balloon couldn't carry a kid. They couldn't have said - sorry no way - and packed it in because what if the calculations were wrong and the kid was in there, even if it was a remote or seemingly impossible chance, etc.
The minute I saw that little balloon land I said no freakin way it could lift a kid.