Thanks for keeping the comments safe!
Its really nice that I don't have to worry about a bunch of swearing on here, because now I can let my little sister use DL.
Don't take this personally, but, I believe you are overly optimistic. Although most discussions are relatively "safe", on the internet (as well as in real life) there can be many nasty discussions, without cursing even a single time. In addition, the fact that duolingo is an online resource, means it can be hacked/defaced and so forth.
So, if you're truly concerned about that, then I suggest you supervise children while they learn. Or wait for duolingo to add supervised accounts, which may have the ability to block certain aspects of the site.