It was invented by the same guy that made the erector set. It was however quite expensive selling for over $600 in today's dollars. It was removed from shelves not due to safety concerns, but low sales.
It came with a Geiger counter, an electroscope, a spinthariscope, a cloud chamber, four glass jars containing natural uranium-bearing (U-238) ore samples (autunite, torbernite, uraninite, and carnotite), and low level radiation sources of beta-alpha (Pb-210), pure beta (Ru-106), and gamma (Zn-65).
It also had a comic book- Learn How Dagwood Split the Atom - an introduction to radioactivity, written with the help of General Leslie Groves (director of the Manhattan Project) and John R. Dunning (a physicist who verified fission of the uranium atom), and Prospecting for Uranium — a 1949 book by the Atomic Energy Commission and the United States Geological Survey.
It was relatively safe, especially compared to other toys on the market at the time. The only warning the kit had was about how the radioactive sources could crumble if removed from their containers and thus mess up the lab equipment.