Pornographic images in app rated 4+

How does someone report inappropriate content in an app? The game Block Craft 3D is rated 4+. In the User Buildings section there is an icon labelled “Tower Haus” which displays a pornographic image. This icon leads to a user village called “Sexy Ville City”, which contains additional pornographic images, including depictions of sex acts. Given the 4+ rating, I thought this app was safe for kids, but unfortunately my 5 year old son and 7 year old daughter found these images easily on their own. I have searched, but have not found a way to report this inappropriate content to Apple, or to request to have the rating changed. It should be 17+. Surely the fact this content is in the “user” section is not an acceptable loophole in the App Store policies, right? The simple fact that users can upload and share content in an unmoderated way should be cause enough for a rating higher than 4+.