If anyone here has young kids that want to send mail to Santa if you send it to the following address Canada Post will send a response back

SANTA CLAUS
NORTH POLE H0H 0H0
CANADA

The UK has a similar program if you send mail here:

Father Christmas
Santa's Grotto
Reindeerland
XM4 5HQ

I know the Canadian one will do international post, not sure about Royal Mail