Facebook is fine for adults with seichel. But “NO” children young enough to be under your care should not be on Facebook! They meet friends of friends they would never meet on their own. They read things they shouldn’t and they write things they shouldn’t. ABSOLUTELY, POSITIVELY, WITHOUT A DOUBT NO!
If your child is already there, you truly have to monitor them very, very carefully and you had better become one of their facebook friends so you can log on at any time and see what is going on with them. That is akin to be in the living room with them when their friends are over!