To ease your concern on World of Warcraft they do have parental options. However WoW is a very addictive and time consuming MMO and players can still be rude in other aspects that can't be controlled by any parental options. And unfortunately in any MMO players can always filterskip any curse filters by substituting letters or abbreviating words. You should take into consideration the age of the child and the maturity of the audience of the game. WoW generally targets teens and up while there are MMO's out there which are targeted to be more child friendly such as Disney's Toontown, another is Nickelodeon's Neopets (www.neopets.com) which as of last time I played it had no real chat interaction between players.
However if you do consider a more involved MMO out of the many I have played I would suggest: Cabal Online (www.Cabal.com), Eudemons (eo.91.com), Conquer Online (co.91.com)
Or you can go to http://www.mmorpg.com/gamelist.cfm for a list of all MMO's to help choose one that is right.