It may have been planned as a publicity stunt.
However, I doubt that he planned on the youngest son having appeared to have left with the craft.
After listening to the 911 tapes of the parents talking to dispatchers, I'm certainly suckered in by either their fantastic acting abilities, or the fact two parents were very worried for their youngest child. This is not to say all the follow on interviews weren't criminal child abuse, nor to say the father wouldn't take advantage of the publicity, but I'm pretty firmly convinced that at least for an hour there, they were sure their child was either in mortal danger, or already lost to them.
Do I think they are being wrongly prosecuted? Nahh, it's pretty clear that releasing that balloon was contemplated in the past for personal gain. And the father's craving for fame at any cost to his family certainly warrants the microscope he'll be under by the prosecutor and child protective services. Hopefully sanity will prevail and an agreement can be reached with prosecutors that'll end the father's attempts to turn his family into a show.
I'm not a big fan of preemptive prosecution, nor prosecuting someone who is innocent of the crime they're charged with, but I'll look the other way in this case.
Oh, I think it was in the plan from the very start. Why else put a kid-sized compartment in the "craft"?