Kids Corner was first introduced in Windows Phone 8.1 and allowed parents to set up their smartphone so their kids could only access specific apps and media. In a blog post, Windows Insider head Dona Sarkar revealed why Microsoft decided to end support for Kids Corner:

After taking a look at the overall usage of the Kids Corner feature in Windows 10 Mobile and Windows Phone 8.1, we have decided to remove this feature from Windows 10 Mobile beginning with the Windows 10 Anniversary Update. The usage of Kids Corner was too low to justify continued development and support of the feature. While not a replacement with the same functionality, you can try out Apps Corner under Settings > Accounts > Apps Corner which has some of the functionality as Kids Corner such as the ability to give people access to selected apps on your phone as well as a customized Start screen when they use your phone.