After all, life itself is inherently risky, so why not buy them the “most dangerous toy in history?” They can learn all about physics, radiation, and the fun of wearing a lead apron (not included) with Gilbert U-238 Atomic Energy Laboratory! But hurry, bidding ends on 12/11!
U238 is about as safe a radioactive material as you can get: It’s got a half-life of billions of years, so a very low rate of decays. I wouldn’t be surprised if you got more activity from a standard construction cinder block.
Also, what radiation you do get will be almost entirely alpha. A lead apron would be irrelevant; ordinary clothes, or even the dead top layer of the skin, provide good shielding against alphas. Alpha emitters are bad if you ingest them somehow, but kids do and did play with plenty of other things that are dangerous to ingest (and in that case, the lead apron still wouldn’t make any difference).
I used to know a guy who still had his set from his childhood. He died a few years back and I don’t know what happened to the set except that it wasn’t willed to me. (Grumble.)