This is... technically... story related? I mean, it is possible to miss these, but the tutorial does spawn up suggesting you get this one.
Chapter 1: As part of the tutorial with the DETECT Truth Bullet, once you leave the hospital you will find your first Hidden Kid. There are five to each chapter, denoted by sparkling lights in obscure places. Given you'll unlock this trophy from unearthing the Masaru icon outside the hospital, you're well on your way to finishing Chapter 1 (and you can keep track of your chapter progress based on the Hidden Kids, too).