This game is like Legos - building blocks. All the games and content you see were created by players, not Roblox company. This means that you can build your own game for other people to play. The games, worlds, clothes, accessories, everything is created by players. Sounds neat. Except that players are people, and a certain percentage of them will always be bastards.
So hypothetically I could create a game that has a huge billboard with an image on it, and I can point the URL of this image to... say... some pornography or worse. There is no mechanism to stop me from doing this, and people have done it. They actually make games to draw young kids in, and then script in porn images to flash in front of them at some point. I've tried over a hundred games and I've run across two of these.
That said, there are still three stars on my review. It's a solid game with a (mostly) good community of kids and parents who play it. If there were some reliable parental controls and monitoring of inappropriate activity, then this would be 5 stars. But since my kids can only play when I can play, that's not very convenient for either of us!
Bottom line: Great game if you can play WITH your kids.
If that is not an option, do not allow your young kids ages to play this or any other online game until they are old enough. Especially sandbox games with player-created content and no controls or monitors.