One thing that a documentary or storytelling photographer will do, is to photograph their subject from many different perspectives.
They can focus on the big picture and the smaller details. The images can then be displayed together ~ either in an album, a wall-art grouping, or a collage (or two!) as shown here. Presenting the images together like this provides the viewer with a more complete story and will help bring that moment back to us years from now.
Think about an activity that your child does that you'd like to remember. You can try this out using a dslr, a point and shoot camera or even using your smartphone.