It will be fun for the kids and it will be dumbed down enough for them if they can relate to it.
As an example of a web server. Have about 10 copies of some childish website printed out and hand them to a volunteer (this kid is the server) then have 10 other kids requesting a copy of the website (the clients).
That explains in general the concept of server vs client.
Then you can say when the said kid (server) gets sick or breaks a leg or whatever, he can't hand out the website anymore and you have to go make him/her better.
I don't think I'd give any tedious details of windows vs unix unless you want to 'enlighten' them but I doubt they'd care.