1. Wait until you see your subject do something interesting, go behind them and take a picture with their back to you.
2. Back side photos work best when kids watch TV, play on their computer, run ahead of you or look at something. In all these situations they probably won't pay attention to you and you can sneak behind them.
3. Photos taken from behind are especially interesting because they give us the point of view of the person we are photographing. If a child is looking at the fish in the aquarium we get the feeling that we are looking through his eye and we see what he is seeing.
Try and find your opportunities to take interesting fun photos from behind!