K9 Web Protection Browser attempts to fill the gap in Safari's access filters by providing a secondary browsing experience for children, combined with adjustment of some security settings. The built-in protection settings on an iPhone or iPad are fairly decent but are in some ways limited, especially for younger children. The Web browser in particular allows open access to the Internet if not filtered in some way. This is what K9 Web Protection Browser strives to fix.
The K9 browser looks identical to the Safari browser built into the iOS design, and while there are certainly some limitations as to what the browser can do, it is a good replacement for the standard browser provided by Apple in that you'll rarely notice the difference. Setup, however, can be complicated. It requires you to change multiple restriction and security settings on the device, and then secure everything so that your children cannot change them back. The process takes a few minutes but once done effectively, it won't allow children to open Safari, and forces them to use K9 instead, which has built-in filters against mature content. The filters are effective without being overly sensitive, so they work well for this situation.
If you are concerned about what your children currently have access to on their iOS devices, K9 Web Protection Browser is a good replacement for Safari. It takes some time to set up, but the detailed instructions provided by the developers make it a fairly straightforward process; and the app is free without ads, which is a nice touch considering the cost of many other browser replacements.