Quoting melindapple:" Holy wow! Idk if this has been posted or not, but I can't believe this little girl managed to drive half ... [snip!] ... her father took her home. The investigation is ongoing and it is still unknown if anyone will face charges (ABC News)."
Kids observe what we do on an everyday basis more than people realize. If you see how your parents start a car every time you leave, it's not hard to figure it out even at 6 years old.
That being said, put the keys somewhere that the kid can't get them.