This tutorial provides the steps for restricting app download based on app rating. For example if you want to prevent your kids from downloading Apps that are rated 12+ then you can do the following
Tap Settings icon on the Home screen
Then tap the Restriction under General Settings.
In the Restriction screen, scroll down to Allowed Content section and tap the Apps option.
Now if you want to block apps that are rated above 12+ then un tick 12+, 17+ and Allows All Apps.