ROBLOX is a very good game, if you know what you're doing. I recommend, if your child is too young to know most right from wrong, then monitor the child's account. If a user's account is detected to be under the age of 13, ROBLOX automatically filters words such as "idiot", "crap", etc. (I think) to make it safer for the younger audience. Also, in the settings, you can change who can message the child, who can chat with the child, etc. If you truly know what you are doing, and you are willing to monitor your child's activities, then ROBLOX is a great game for them to open up their imagination. I suggest, if you want to make sure your child isn't playing games that they shouldn't be, e.g. shooter games, robbery games, etc. then every so often, just for a few minutes, go on their account, and on the home page you can see their "Recently Played" games. To see all recently played games, you can click the button to the right of that title that says "See All".
Overall, I believe ROBLOX is a great game for children to play, as long as they are mature enough to know right from wrong. And if they are younger, then I'd say it's still good for them to play too, as long as you are monitoring their activity on it, and you make sure they know not to play games that you wouldn't approve of, and you can in the settings change who they are aloud to interact with.
Also, a side note, ROBLOX has a parent page here: https://corp.roblox.com/parents/
I highly recommend reading it, as it tells you lots of important information.