I myself am a huge sucker for anything about the kids, made by the kids, or picked out by the kids!
I LOVE Christmas pictures! For most of us who watch the same children on a regular basis - they become like family and we have a special bond. Having pictures of them at home keeps them close to us when we're not with them. We also get to show off how cute your little ones are when guests come over!
They also have those ornaments you can buy that record voice messages. If you child is talking, he could record a cute message for her. Or you can go to a craft store and buy an ornament to be painted or colored and let him do it.
Another option, if she is at your house when she sits for you - make her a gift basket of her favorite things (crackers, hot chocolate, chocolates, etc) to stay at your house, to make her feel a little more at home when she's there.
I'm not sure what kinds of things you make - but you can make those glass block christmas light presents. Cut a hole in the glass block with a diamond bit - insert a small string of white christmas lights and spray frost on the block. You can also decoupage a picture onto them printed on paper - to make it more personal.
Whatever you make/give her - have your son color on paper if he's able, and use that as wrapping paper. It's adorable and gives that warm fuzzy feeling! If you'd prefer to bag her gift - you can get plain paper bags and have him color it, too.