Drop 'em an email. "How are you? Did the kids have a nice Christmas? What was the best gift? Our favorite moment was hearing some carolers a block away. It was sort of a neat feeling--eavesdropping on someone else's wonderful moment, sort of sharing in the mood."
You could, I suppose, say, "I heard about all the delays with shipping this year, it was even on the news. I hope our package got through."
Just be in ordinary touch, only perhaps a little more frequent.