Southern Wolf, here are a few tips I can offer from various shoots I've done in the past that had kids in them.
THe first and most important thing to do is get comfortable with the child or children before the shoot. Having a kid look scared in the photos is going to be your worst fear for ruining your images.
What we usually do is get there an hour or two before the shoot is going to start, and sit with the child in a casual manner. Tell the parent to have the child take a nap an hour or two before you get there, and when the child wakes up, maybe have a lunch ready where you can all sit around casually getting to know one another. I usually try and get to know the kid before hand like having a meeting with the child and the parents just to talk about location and what to expect.
Another thing is to have your camera out and in your hand the entire time that you're around the child. When I first started shooting people I didnt know this and pulling the camera out just before the shoot starts, usually freaks the kids out and tenses them up. They suddenly have this strange man pointing this big oject right in their face.
So have the camera in your hand the entire time, and what I do is usually shoot a few images, and show the child the results on the camera so they know what your doing. Having the child fully relaxed will get you the best shots possible. This is why I ask the parent to do the shoot in a location the child is very comfortable with, like their own backyard, or a playground where they know and like to be at.
Having the parnet behind you, along with a sibling or someone who can keep the child at ease will also help. If the child is still standoff-ish, try and take your first 20 to 50 shots with the child and the parent together, so the kids know nothing bad is going to happen.
Tell the parent to bring a few outfits and some props. Kids love props....hats, scarfs, sunglasses...etc. The more you can get the kid to act goofy, the more you can get more natural pictures.
Once the kid shows any kind of tiredness, the shoot is over.
I know I've gotten some of my best shots where I told the child and parent to just play naturally in the back yard, and I just followed them around shooting 100% candids. Every now and then I might ask the child to look my way, but 99% are just capturing the child being a child, and the parents have always loved the results.
This was a shot I did for a day care center. The rest of the kids were out playing and this little angel wouldnt have any of it. Like I said, some of the best images are the ones you cant pose.
I have a good firend who does this for a living, child photography specific, and she almost only shoots candids of the families playing in their own surroundings. Her work is amazing. www.sonyaprather.com
and she makes very good moeny at it and has a waiting list of clients who want her work hung on their walls.
Kids are one of my favorite subjects, because they're just playing and acting natural, and that's what is so timeless about them!