It may sound an obvious thing to say but I find that photographing a whole family is much harder than photographing an individual member of that family. A group photograph demands a very different approach as you may well be dealing with people who have various reasons for wanting (or not wanting) to be there. There can be rivalries between siblings to deal with. I have had situations where one or other of the parents has driven the shoot against the wishes of the other partner - in these situations it is usually the wife who has taken the initiative. If there are big age differences between the children this too can present problems when trying to set up a balanced photograph - and very young children often have a very unnatural attitude towards being photographed, forcing expressions such as smiles so that they look false.
My way of dealing with this is to encourage the family to have individual photographs of the children taken first. This allows me to establish a relationship with each sitter separately during which you try to establish a trust with them and emphasise that the whole process can be fun.
Once these have been taken, then I gather the family together. The most recent shoot I have done of this kind was in a beautiful garden adjoining a small lake. I had plenty of backdrops to exploit and the light was pretty even although, as the late afternoon moved towards evening, a bright sun emerged. At this point I placed the family under a delicately leafed tree in dappled light. By placing them carefully and making this whole procedure an open discussion they began to feel part of the creative process rather than passive 'pawns'. Once we had settled on the grouping it was far easier to sustain a light-hearted atmosphere which, in turn, made their expressions even more natural and, importantly, interactive. Banter loosened everyone up and the result was an unusual but very satisfying portrait of a family that had almost forgotten they were being photographed, yet one that had maintained their discipline.
I am always apprehensive about family photographs but when they work, they are great fun, and capturing that sense of fun is partly what makes the final image successful.