The titles included in the “Just for Kids” library come from Common Sense Media ratings, determined safe for kids under 12 to watch. Of course, parents should always be aware of what their kids are watching regardless of the kid-friendly interface. The only way to really limit your Netflix account to certain ratings is to create a separate account.
Netflix says at least half of their 20 million users have watched at least two kid-friendly movies within the last two months.
Source: Netflix