You could take a picture or two of each of the kids with their favorite things (favorite toy, book, people, pet, food, etc.), and then say something like "Wishing you a holiday season full of your favorite things..." on the card
Or if you're good at capturing candid pictures, you could try to get some shots of your kids with different expressions (laughing, daydreaming, etc.) and do a card with a caption under each photo like "Wonder," "Joy," "Laughter." Then you could put under that..."Wishing you a holiday season filled with all of the above..." or something like that. (Hope that made sense...!?)
Good luck! I'm trying to some up with some new ideas myself.