I did a really cute one last year with my twins. It's my favorite we've done so far. They were 4 years old. I placed a white sheet on the floor (to serve as the background), and put them in white tshirts with santa hats on their heads. I had them lay down really close to each other on top of the white sheet, and covered them up to their necks in a white sheet. So, the only colors in the pic are red and white. I then gave each boy a huge peppermint candycane, and had them hold it just at the edge of the sheet (like the kids in the Grinch cartoon, just before the Grinch steals their candycanes). Then I had them close their eyes and snapped the pic. It was adorable.
When they were around 2 1/2 y/o, I baked cookies, then put them in trays on the floor, along with bowls of red and green icing and sprinkles. I put the kids on the floor with it all, in just their diapers (I put white diaper covers over the diapers to make it look nicer), then I let them go to town and started snapping. I got a great shot of them covered in icing, both looking up at my dh...they had a look on their faces like they were caught doing something wrong. I put a caption on the pic that said " Cookies??....What Cookies???..." that was a cute one, too.
Another year I just put them on the floor in front of the tree with some wrapped gifts, wearing red and white striped pjs, and gave them a big roll of gold ribbon. I had them throw the ribbon in the air a few times and play a bit while I snapped pics. I chose the best one for our card.