Just in case you didn't know about it, I am writing about the Santa letter-writing program from Canada Post. Your child can pen their letter to Santa, include a return address, and receive a special letter back!
You should mark the letter with this address:
North Pole HOH OHO
You will receive a reply from Santa, courtesy of the helpful postal elves.
Alternatively, you can also email Santa from this page, and you will also get a response there.
When I was small, I never actually mailed any letters to Santa. I usually just wrote it and left it out for him on Christmas Eve, beside the cookies (which were always gone by morning)!
I think this is a really nice gesture from Canada post though, to take the time to return all the letters. They have been doing it for 29 years!