What's in a part?

I am often puzzled at how to part my girls' hair.  I am a big fan of the side part with hair swept to one side with a small (not over-sized) bow holding it into place.  I think a bow put in a child's hair should be proportionate to the size of the child.  Nevertheless, when the hair doesn't naturally fall to one side or the other, which way do you part the hair?

With my little one, I have always parted it to the same side as her sister's....(thinking...wouldn't it be cute in pictures if their hair was parted the same way and styled the same).  Recently, her hair has taken on an enormous cowlick.  Even with water it doesn't lay flat.  I was describing this problem to my mother yesterday and she promptly removed the bow and combed the hair to the other side.  VIOLA!  The hair laid flat.

The wisdom of Grandmothers amazes me sometimes.

Do you know the proper way to find a part?