In reply to Denni:
I don't think you're asking one question here, you'r easking two.
Which do you want to explain to a 6 yr old?
Employees having expenses is easy. I get to claim expenses if I travel down south because otherwise my wage could disappear on train fares.
Further, MPs HAVE to work all over the country but also HAVE to be in London some of the time. They need to be given accommodation and travel expenses or no one would be able to do the job who lived far away and werent' already rich ( and willing to spend that money on their job).
Why are expenses abused and possibly unfair? another question entirely but I think it's colouring your approach here. Do you want to explain political corruption and all that goes with it to a 6 yr old?
ps. smart kid.