I completely understand your concerns, as a parent it's natural to want to keep your child safe online. FB Messenger can be a great way for kids to stay in touch with friends, but it does require some parental oversight. First, you should know that Messenger allows users to control who can message or call them, and you can help your child set their privacy settings to "friends" or "friends of friends" to limit who can contact them. You can also show your child how to block someone if they're being harassed or bullied, and report any suspicious activity to Facebook. Additionally, you can consider setting up a family account and using the built-in parental controls to monitor and limit your child's Messenger use, such as setting time limits or restricting certain features.