Try a collage for blended family's card
It is understandable that you haven't been able to achieve this, leaving me to wonder about friends who would receive this card and then choose to make a sarcastic crack about one of your children.
Photo cards are great -- and the options of how to create them are almost endless.
I vote for a "collage"-style photo illustration, which would picture each family member photographed individually. Maybe you could photograph each child wearing a Santa hat; your oldest son could very easily photograph himself and email the photo to you.
You could opt for something more sophisticated, but my holiday tastes tend to run toward the nuclear Santa hat option -- including the family pets, of course.
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