Netflix is an online subscription service that provides access to television programmes  and films. Their terms and conditions state:
'4.1.   You must be 18 years of age, or the age of majority in your province, territory or country, to become a member of the Netflix service. Minors may only use the service under the supervision of an adult.'
If you allow you child to use Netflix it must be done so under your supervision. As part of your membership, you are able to set up different users within your account. One option allows you to create 'Kids' user' with restricted access to content. 
Netflix contains films and television programmes from many different genres including films that are only suitable for adult viewing. If you allow your child(ren) unrestricted access to a Netflix account, you are potentially allowing them access to these items.
There's lots of advice on how to set up a Kids' user and how to PIN protect your own user profiles. Here are a link to a few useful sites but there are many more: