I'd suggest getting the kids involved. (Score 1)

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.