Slightly away from topic, but here's some ideas/techniques to try with max aperture (range of f4 to f1.8), and portrait lens (focal length 85-200mm), for child photography;
1) Soft directional light, e.g. open some french windows or a door, place subject just inside so no overhead light. The background will probably be much darker (as the light falls off the further into the house/room you go). Or under a tree on a sunny day, or on a cloudy day in the open, or indoors close to a window (windows can make lovely frames for photos, even shooting from outside through the glass).
2) For children, get them focussed (every pun intended) on something, like a game (try stacking stones on the floor), or talking to another adult/parent, or picking a flower, or doing some homework, anything to distract them from a camera.
3) Think about colours on subject and background - e.g. blue jeans and plain t-shirt, whilst surrounded by yellow flowers and green grass.
4) Look for interesting textures in the background - if a garage door is a nice colour like french blue that's great, but if the paint is peeling that's brilliant, the texture can look awesome, especially in b&w.
5) When summer comes, take her into the woods (have another parent/guardian looking after her, so you can focus on the pictures), and turn it into a fairy hunt or similar. Plant a fairy door maybe for her to find - the expression that will give on her face would be natural and priceless, you just have to be prepared to capture it. Using the thumb focus can help with this, as you can pre-focus and be ready for the action (without the camera focussing again when the shutter is pressed for the picture), but it takes a little practice.
