My daughter (almost 9) has been playing for a while now and recently started going into online servers.
I allow her to while I am in the room so that I can keep an eye on the chat section. Her favorite server is play.emeraldisle.net. They have their own plot to build and only people they have invited into it can build there. No matter what server they go on there will always be not so nice kids of different ages. My daughter has a few online friends that she meets with in the game.
The safest one I know of is Scrafty, it is a Christian based Minecraft homeschool site with strict rules. I know they allow non homeschoolers in there you just need to sign up.
http://whenyouriseup.com/
We use it for some classes.
Remember to always be with your child while they are online to be sure they don't give any personal information.