Block messages except from friends, make sure they turn off chat, etc. There is no way to prevent them from looking at the forums and they can always change the settings.
Under 10 probably should not be hanging out at chess.com, that is the purpose of chesskid.com.
I use chesskid.com extensively for my for managing my current group of students and allowing them access to puzzles and games.
I am planning to introduce accounts for some of them on chess.com because they clearly demonstrate a higher potential than typically that of the average rating on chesskid.com.
I am concerned though because these kids are below 10 and I don't want them to be victim to any inappropriate chat or receive unwanted messages to their inbox. What can be done to prevent these instances?