1. The content (What is the child really looking at? What are the features?)
2. The context (What is happening around the child at the same exact time they are looking at the screen? Are there other kids around playing? Is no one else around?)
3. The child (What is their needs? Do they have any stressors in their life?)
So if your child must interact with apps, School Library Journal is the only place I know of that reviews apps for kids. Their Top 10 Apps of 2013 is a good place to start if you want apps for your child to use. The list includes a Mo Willems and a Captain Underpants app too!