They may seem harmless and look cute, but juvenile turtles carry the risk of Salmonella transmission.  Recently, police in Baltimore seized dozens of tiny turtles from vendors who were selling the animals illegally.  The sale of turtles with shells under 4 inches in length has been illegal since 1975.   According to the CDC,

This size was chosen because small children are more likely to treat smaller turtles as toys and put them in their mouths. The ban by the U.S. Food and Drug Administration (FDA) has prevented an estimated 100,000 cases of salmonellosis each year in children. Despite this ban, such turtles are available from pet stores, flea markets, street vendors, and online stores.

The CDC has also issued a report  of a  2007-2008  outbreak of Salmonella linked to turtles.