Please read the available material published for parents by Roblox. I wont go into too much detail or post specific links, as the advice and information changes quite frequently. Currently, if you visit help.roblox.com there is a section Parents, Safety, and Moderation which details everything you need to know.
As a parent that plays Roblox with two children under the age of 13, I've found that the in-game chat filter is pretty good, if anything it's overzealous. Your child wont be exposed to any comprehensible language worse than anything they might be exposed to in the school yard.
However, the big thing to remember is that anyone can submit games and other content to Roblox which your child may be exposed to before it's been fully reviewed by Roblox staff if you don't ensure that the account has the recommended restrictions in place. It is feasible that your child might stumble upon a game that hasn't been reported and blocked for containing inappropriate content if you don't ensure the account settings will prevent this. This is especially important if you are considering allowing your child to play unsupervised and they like to explore for random games.
Currently, Roblox provide parental controls and account restrictions which includes a setting to only allow the account access to games which are considered appropriate for ages under 13. These settings can be locked with a pin. If you enable account restrictions on your child's account, Roblox is reasonably safe. I'd also recommend reviewing the privacy settings on the account.
If you want to be sure if a game may be appropriate specifically for your child, I highly recommend jumping on and playing it first. Or play it with them and coach them through the experience expressing whatever values you hope to imprint on them. Some games are really dumb. For example, some games are essentially just clicking a button on the screen as fast as you can which isn't only pointless but also bad for them and the mouse. But hey, they seem to enjoy it. If they don't lose interest in that after an hour, download an auto-clicker for them or introduce them to Progress Quest. Many of the games are great fun and in some cases it can be quite educational for them.
There are also scammers on Roblox that try and trick kids into revealing their account passwords, promising free Robux. Make sure the account is tied to your email address, specified as the "Parent's Email address" so that you may easily recover the account if your child falls for such a scam.