Here's a picture of one of the cards my sister in law often sends to my children:
Her husband died, and she has just enough funds to live on; buying gifts is not an option. She's very attentive to birthdays and other events in the children's lives. She sends these hand made cards, and a couple sticks of gum or a few tea bags. The kids love getting mail addressed to them, especially with a little treat inside. I know it costs her time; but compared to other gifts, the monetary cost is small. It's disproportionate when compared to how much they like it.
Most of all, they can learn that what really matters is the care and attention we give to others.