Here's how to do it:

1
Go to Manage Profiles and click on your child's profile.
2
Under Maturity Settings, click Edit, and enter your account password when prompted.
3
Choose the maximum rating that you'd like to appear on your child's profile.
Recommended for Adults\n\n May be inappropriate for ages under 17. TV-MA. For Mature Audiences. NC-17. Inappropriate for ages 17 and under.