Expression over Perfection
In order to get great, natural expressions, you need to build a relationship with your subjects, talk to them beforehand and throughout the session, praise them when it's good and ignore the bad. Develop a rapport with them and be genuinely interested in what they have to say. It’s important that you relax, keep the conversation light, be yourself, and allow the connection to develop naturally.
With children it's all about play, running, jumping, anything goes, and then you're much more likely to get great expressions, like in the above photo.
Seated poses can sometimes be easier
I asked her not to sit down flat on her bum, but instead to roll onto her hip, that avoids her lower area 'squishing out' (that's a photography technical term!!). Also notice the way her arm is draped over her loving husband's knee, which shows the contact and relationship between them.
Use different anglesWe see the world from our own viewpoint and height (obviously!!), so changing the camera angle or the camera position can have a dramatic effect because it's not how we usually see people and scenes. In this photo, taken outside in a park, I asked this pretty young lady to sit on a step, and I stood on one of the higher steps. I made sure I wasn't too close to her, as (apart from invading her personal space) getting in too close can create unwanted distortions.
I used a fairly wide aperture for this photo, around f2.8 to blur the background, and so kept the emphasis on her lovely eyes.
Try spreading out large groupsThere's no law that says you have to put people in a line or in rows when posing large groups. Instead try positioning them separately into their respective smaller family groups with maybe some standing and some sitting. It looks better if they are not all doing the same thing or facing the same way, and gives more variety to the overall image.
Notice the way that some of them are leaning or resting against the pillars or steps, people tend to find that more relaxing than standing in an open area, and it tends to lead to a more natural looking pose.
Heads at different heightsYou can add impact to any group pose by ensuring that their heads are at slightly different heights. If you get it right, sometimes the heads can form diagonal lines (like in the above photo) or shapes, which again can add impact (see Use diagonal lines for better photography composition).
Notice I've asked them to lean against the railings, as I knew that would look a little more natural, and be easier for them whilst holding the baby. Also, I've positioned them with their backs to the light to provide some rim lighting.