It's the users not the game.
I've been playing this game for a long time, before gems came into play. I would say any website with chat, avatars or anything of that nature are never safe for kids. Because anyone can play them and some people enjoy ruining things for others. When "hearts" were created I had a side account just so people would stop asking out of the blue. It was suppose to be something serious and the immaturity of the players was ridiculous. Girls claiming they were cheated on and would openly chat about suicide, That's when you know they take the game too far.
The second issue I found was the levels, once kids reached a certain level they became arrogant and rude, I always found the term noob rude and senseless, everyone was new at some point. Higher levels always bashing new players for being "ugly" because they have starter outfits or clothes that didn't cost gems, or simply making fun of them for asking a question about the game.
Then gifting, with it's constant begging, and some kids trying to scam new players, into buying them gems or something of the sort. I could log on and will find someone I don't know commenting me to give them a gift. It gets to a point where you don't even want to leave your profile open to all players, for anyone could message and comment spamming about gifts, or my next issue.
Kids who try to talk or display sexual actions.Being naked means getting down to undergarments. But people will use their imagination and do dance moves to interpret sex. OW did block a lot of terms from being used but you can say anything if you know how to get around it.
Lastly, anyone can play it. But if your kid wants to make an account you better hope they're mature and know the difference between real life and the internet. It is not the games fault that a variety of ages play this, and it's certainly not their fault when users want to misbehave and act rude or crude. So there really isn't a way to shelter your kids from anything, this happens everywhere.