Prosecutors say a Montgomery County Public Library employee blackmailed hundreds of children into sending him nude pictures of themselves through Facebook.

Investigators say 37-year-old Timothy Alcala was preying on minors, many between the ages of 9 and 12. This is important because Facebook says you have to be at least 13 to join.

How these kids signed up may have put them more at risk.

If you allow your pre-teen to falsify their age to open an account, make sure they are picking a birth year that still makes them a minor.

Facebook has some built-in features that provide more privacy for minors like preventing them from receiving messages from strangers. By default, a minor's account is automatically set to allow only friends to see their information and posts.

Make sure your child hasn't switched this setting to "public" that will allow everyone to see their activity.

Location sharing is also off for minors unless they turn it on. And Facebook blocks sensitive information, like a minors' contact info, school and birthday, from appearing in searches to a public audience.

Prosecutors say Alcala used "alias" or fake Facebook accounts to reach his victims. That's why it's important to tell your kids they need to know everyone they accept as a Facebook friend.