[quote=@Rilla]
Because, in general, people don't like their property getting away from them. And slaves are generally put under the whole 'try to escape and you die/get tortured' contract. And owners really like enforcing those.

And guards really don't wanna get punished for watching a kid escape. 
[/quote]

I think the point was that once she got out the arena, the guards wouldn't do anything to stop her because they don't know the child is an escaped slave. They're just walking around town doing their normal guard related activities so don't pay a lone child a second glance.