It doesn't need to get expensive, as you can use the scrap post as "fancy paper" for your cards. Also, your kids' art - and your own - is absolutely adequate as card decoration. It's at least better than sending some store-bought picture-perfect and soulless cards.
Monica Dunkley's children's artwork :-) I love these :-)Again, there are tons of ideas for card making on-line.
The postal fees might be horrifying, but you could take a day off and deliver the cards yourself with the kids, as a holiday tradition.