When it comes to photographing toddlers, the fact that they can run away from you makes photographing them a little more tricky. I have found that using a prop can be a great help. I like to have my camera and tripod set up and ready, and then mom or dad can draw the toddler's attention to a toy or prop that is strategically placed in front of the camera. When they run over to grab it or investigate further, you can be ready to snap. Using a tripod this way can really help capture the perfect smile when it happens.
Once children become a little older (4-10), photographing them can be easier in some regards and harder in others. The key is finding an activity that interests them so that they fall into a pose that works well for them and looks natural at the same time. I worry less about the “posing” aspect of this age group and more about keeping them occupied. You’ll often find that the best poses are the ones they create themselves. You just have to be ready when it happens because it doen’t last long!
High school aged students can be tough in regards to posing as they may want to go in a different creative direction than their parents. In other words, it can be just as difficult to get an older child to smile as it can be with youngsters. While parents may want smiles in every shot, many high school students imagine themselves being photographed with a much more "cool" expression. I try to cover a mix of both styles to please both my subject and their parents.
Like with adult subjects, the key is to build a rapport and help them to relax in front of the camera. If I am having an especially tough time getting smiles, then I like to set up my tripod and ask my subject questions about their interests and hobbies. Usually when people speak about something they are passionate about they can't help but smile and look engaged. If you have your tripod ready to go, you can capture these moments as they happen without your subject feeling too pressured.
With the camera on a tripod (using a remote or cable release) you can make eye contact and this makes building a connection easier. I also like to use humor when teenage subjects are having a tough time. Self-deprecating jokes seem to work the best with teens.