"Think outside the box." Or just think "of a box." It's pretty simple to make small boxes from old Christmas cards.

Step 1: Card

Take one used Christmas card.

Step 2: Cut

Cut the card on the crease.

Step 3: Fold

Fold the picture side (this will be the top of your box) in half, but only pinch it on the end where the arrow indicates, do not crease it all the way down.

Step 4: Mark

Open the card and put a small mark where you pinched it.

Step 5: Fold and Mark Again

Fold it lengthwise and do the same thing as the previous step.

Step 6: All Four Sides

You should have all four sides marked.

Step 7: Fold Everything

Fold up to exactly your pen marks and crease, I used a sharpie to really push it down. Fold, crease, and open all four sides.

Step 8: Cutting

Cut from the edges where arrows indicate.

Step 9: Fold

Fold the sides in.

Step 10: Tape or Glue

Tape or glue the flaps down.

Step 11: Bottom of the Box

You want the bottom of the box slightly smaller so the top will fit over it. You'll do the bottom almost exactly the same as the top except you want to make your folds slightly over your pen marks so the bottom is a tad smaller. I've slanted my fold so you can see how far you need to fold over your pen line.
You then mark, fold, and cut the same way as the top of the box.

Step 12: The Box

When you're done the boxes should fit together perfectly.

If you want your box to look not so creased and a bit more professional then I suggest measuring instead of pinching the folds. You could make a stack of these boxes and give them to the kids or you can fill them up with small Christmas gifts for next year :) Have fun!