I think the background is appropriate for a Christmas card. I think the shot still needs some work. The background is brighter and sharper than the portrait, making the girls look a little dull. I'd like to see the background toned down, and the eyes and teeth sharpened more. Maybe even see if you can brighten the blue in their eyes. It's a nice shot of the cute kids. You captured some good expressions.
I'll differ with Jeffrey - I think for Christmas cards, digital backgrounds - especially this one - work just fine. This is bright, cheerful, and a GREAT shot of the girls! Makes me smile. Very good job, in my opinion.
Sweet shot of the kids! I'm not crazy about digital backgrounds, though - they always look fake to me. Maybe you could buy a disc of stock photos to use instead?