You know Christmas card picture taking is not going to go well when it starts out like this, before you've even taken out the camera:
So, you get one calmed down, and the other decides his life is pretty horrible, too:
So, you get him calmed down, and then the first one decides she's not over whatever she was throwing a tantrum about before:
You just decide to scrap the whole thing and try again the next day. Because once the girl decides that whatever you're doing is not OK with her, you can forget talking her into it.
As you are excitedly reviewing the new pictures because they were both in good moods, you realize your children are either a) possessed by the devil, or b) just very, very odd.