Have you heard about the newest metals? They have been given temporary names, Ununquadium and Ununhexium.
The dust that makes up fallout is so fine and high up in the atmostphere that it doesn't all fall hours, days, or even weeks after a nuclear blast.
Both kids and radioactive material need supervision. Like a child, an energetic radioactive molecule will become less energetic as it ages. Certain materials, like those used in radioactive medicines, have only low-level radioactivity and will decay in only a few days or months.