How would you explain what is a race condition to a five years old
A lot of race conditions result from improper or unexpected interleaving of processes. Someone that age can probably understand the fable of the Turtle and the Hare. 

Give the Hare a ball with instructions to run to a table, some distance away, and leave the ball on the table and return. 

Tell the Turtle to go to the table, pick up the ball, and return with it. 

Start the two "running" at the same time. 

The Turtle has no instructions about waiting, since the proper interleaving is obvious. 

Of course, if the Turtle gets to the table first then the process fails. 



I'll note that Dining Philosophers isn't a lot more difficult than the above, but probably not accessible to a five year old. Certainly open to one a few years older, though.