Click on your profile picture in the top right corner of the screen to reveal a drop-down menu and select Account.
Scroll down to the bottom of the screen to the Profile & Parental Controls section, where you'll see a list of all the profiles on your Netflix account.
Select the down arrow to the right of the kid's profile, scroll down to Viewing Restrictions on the list of options, and select Change.
Enter your password when Netflix prompts you for it, and you'll be taken to a sliding bar that you can extend or shorten based on a title's rating -- from TV-Y to NC-17. You'll also be able to enter specific titles in the field just below the rating restriction bar.