There's an old saying in Hollywood, never work with kids or animals. For photographers there's rarely a choice.
The problem, as any parent can tell you, is that it's difficult to get children to follow even simple instructions. Start telling them to turn their head three-quarters to the left and bring their hand up a little bit on their waist and you're likely to get a lot of pictures of a very bored or very frustrated child. Approaching a shoot with children like you would with a model is a ticket to failure, here are a few tips that you can use whether you're a parent or a pro.
1. Get on Their level:
Higher angles tend to diminish subjects in a photograph, pure and simple.
There's a second advantage to getting down on your knees or your belly, it opens up a whole new perspective. This is the world a child sees, a world that most adults have forgotten. Just as things shot from higher angles are diminished, things shot from below are embiggened (which is a perfectly cromulent word) which can add a sense of wonder and excitement to a photo.
2. If you don't know the kid, bring an expert
Knowing the person you're photographing is invaluable, but when you don't have the time you can fake it. Parents know how to get a child on task and how to get a child to laugh or smile. It's great when you're starting a shoot and everyone's nervous and you have yet to start a rapport.
A parent can also make a child more relaxed and comfortable, which produces more natural photographs.
3. Let them play
Kids are at their best when they're relaxed and having fun. In a studio setting props and toys can be a great ice breaker, but when you're on location there are so many things you can do to encourage a child to get up and move. Not only do the smiles come easier but it creates a much more dynamic portrait.
Your first few shots with a new subject aren't always going to be great. You're getting a feel for your subject and trying to find the right story to tell, so don't worry too much about the photography at the start of a shoot. Take the time to set the mood you want and observe because it will make for a better product in the long run.
For the casual photographer you don't really need much. The biggest problem with children is that they're always on the move and most point and shoot cameras can't keep up with them. A great gift to a new set of parents is an entry-level DSLR (or a high end point and shoot, one that doesn't have a shutter delay) and a fast 50mm lens (which can be had for less than $80). With just those two things you'll be well equipped to capture the moments of a child's life.
The key to great children's portraiture is about unlocking what's great about childhood. There's a sense of magic and exploration, if you can let that carry through the frame then you're off to a great start.