One of my clients last year brought their children in for Christmas cards wearing their Christmas pajamas. She didn't want to bother buying them new Christmas outfits, and I think it came out really cute.
How about you put little aprons and/or chef hats on the girls and take a shot of them making Christmas cookies? Gingerbread people would look great! Good luck!
Have them lay down in the floor with their heads pointed to the center, then stand in a chair above them and make the picture. You'll have 3 heads in a circle.
Last year I took my then 7 year old and 3 year old, myself and husband and used our thumbs--dipped into the black ink and put on card stock paper, added lines for antlers (brown) and used a black marker for eyes and red marker for the nose and made reindeer. They were a big hit for all our family members and friends!
Here is a page on ThriftyFun that has lots of clever photos ideas.
Try dressing them up as "nutcrackers" or elves, etc. Or you could dress each of them up as Christmas trees.
Hope this helps.
Here are two websites that might give you some ideas.
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