pkgExp, Package Explorer is tool for programmers to explore information about what Android apps do on there devices and how they are configured. It displays information about the .apk files and what Intents, Permissions, and Activities they declare. This information can be used to understand what Services and Activities an app provides, what Intents the app responds to and what Permissions the app asks for.
No data, no user information or device or app data is transmitted from your device, sent over the Internet, shared with others or stored -- unless the user explicitly does it. In the future the app may store reports, tables of information, uncompressed copies of AndroidManifest.xml file, on the device so that you (a developer with 'adb' or 'ssh', etc) to upload.
This app does not target children, unless they are Android Developers or computer science students. The only sense in which the app contains Mature Content is that you need to be pretty old to be interested in programming design and security information of apps.