I'm a bit confused. So they really didn't know the kid wasn't in the balloon?
It's heavily implied that the writer thinks Mayumi was telling the truth in her written account, despite what she and Richard said when they called back: that Richard asked Falcon to hide in the basement, and when he wasn't down there, they were legitimately concerned that he could have actually been in the balloon.
> According to Mayumi’s notes, all of that had been an elaborate ruse. Now, it appeared, they were doing it again, this time for an audience of one.
He asked his kid to hide in the attic, but instead the kid hid in the basement.
When he couldn't find his kid in the attic he assumed the kid was actually in the balloon, so he freaked out and started calling the media and authorities.
I'm not sure whether he originally planned to get the authorities involved or not. In any case he planned a hoax but it backfired on him big time.