You'd need thousands of those button sources to make a lethal dose of Po-210.

http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/po...

> They are typically in a form that, if ingested, would pose no health problem, and the radiation is so small that they do not pose a hazard even if the polonium were to be absorbed. It would take 30,000 of these exempt quantities to represent the 3-millicurie fatal dose estimate.

Button sources are typically of the size where you wouldn't mind letting undergrads gnaw on them during their physics labs. The worst they could do is choke.