I love this pose that Krista caught.  Putting the children up front and the parents blurred out in the back (with big smiles) is always a winner when it comes to family photo ideas.  For this shot, since the two girls are not exactly on the same plane, I would have shot this between 2.8 and 4.0, depending on which lens is used.   And the emotion on the older girls face is an added bonus here.