I have created some little colour in postcards and they are FREE to download!
They are perfect for your children to colour in and deliver to their friends.
These are going to be best printed onto some card-stock (If you're a local then Medowie Newsagency have some down the back near the PO counter) and you can just feed that through a standard ink jet printer.
If you don't have a double sided printer then you could print the covers out first and then reload into the printer to print the back (just do a little test first to make sure you know which side you printer prints on!)
If that's too tricky then just print the covers and let the kids write their message on the back.