To Pose, or Not to Pose?
Well, here's a can of worms for me to open: Should you pose your kids to shoot video of them? Or just shoot candidly, and hope for the best? Of course, each technique has advantages. I'll just say that having a mix of posed and candid shots in your collection will probably give you a fun archive—so go ahead and shoot lots of both.
Despite my unwillingness to provide a conclusive answer, I'll happily address some considerations on the posed-versus-candid question.
One obvious starting point is whether the child in question wants to pose or perform for the camera. Some kids paste on fake smiles, some won't talk, and some won't notice a camera at all as they continue merrily along their way. If your child is nervous when on-camera, don't make a big deal of it. Situations that turn into performance that can be judged won't make him or her any less nervous—in fact, quite the opposite. Also, consider the situation. Are you shooting footage so that you can capture a sense of what your kids were like at a certain age? Or do you want to get them saying "Happy Birthday" to Grandma in a voice loud enough for her to hear?
One of the downfalls of shooting candidly is its unpredictability; you're subject to your child's behavior and your own, potentially amateur shooting. Of course, that's also the upside. Most hilarious childhood moments can't possibly be scripted, so if you want to catch the humor raw, shooting candid is mandatory. While you may not have control over your child's behavior, at least practice will allow you to control your part of the equation. That means practicing your framing techniques, as well as understanding how best to use natural light. Furthermore, on the iPhone, you'll need to be especially aware of audio (more about that shortly).
Luckily, getting practice on the iPhone is easy; it's simple to whip out, and just as simple to delete clips you don't like. So shoot lots, and toss lots—practicing is easy and valuable. While you're in practice mode, remember to consider why certain shots didn't work: Were you standing over your child? Was the lighting off? Was the video too shaky? With a little consideration of what is and isn't working, your candid shooting will undoubtedly improve.
The obvious upside to capturing posed shots is the time it gives you to prepare either yourself or your child for the shot. Unfortunately, that time brings with it the possibility of your child becoming nervous, bratty, or unnatural on camera. Fortunately, filming with the iPhone is actually a wonderful way to help children get over camera fright—it's an easy device to pull out, so the more often you do it, the more likely your kids will get used to it. Furthermore, it's a low-fuss camera, compared to some camcorders; chances are good that if your kids are anything like the kids I know, they already love playing with it and watching videos on it.