Draw (or have your child draw) a simple tree shape (a triangle over a small rectangle).
Cut construction paper (scraps left over from some other project are perfect) into tiny pieces, the smaller the better.
Starting at the bottom of the tree, apply glue row by row, put the paper pieces on, apply more glue, keep going until you're done.
Draw a tree-topper or make one from paper pieces.
Scribble a short note inside the card (write it on a piece of paper so your child could copy it).
Make as many cards as you need. Mail them to friends and family.
And here are the cards we made last year (with almost-4-yo.).
Scribble a snowman (or a Xmas tree). Use the stick-on foam mosaic pieces to decorate it.