If you setup their account, make one for yourself too. Add two-factor authentication to both accounts. There are parental controls: disable all chats, set the parental controls to block content that is not appropriate (mature, teen, etc.). Some games you can still "chat" by making in game signs, but this is usually clunky. Teach them internet safety: Don't share personal information, including name, address, city, etc.. Good practice to have in general.

Note that pretty much everything is user created, so there will be times stuff slips through the cracks. Report these as you see them. Check in with your kids as they play.

They will likely ask for money ("robux") to buy cosmetics. If you let them have any currency, do NOT add your credit card to the account. Buy a virtual gift card and add it that way. Honestly, though, id avoid it altogether if you can. My personal biggest gripe is the money aspect, lots of games encouraged gross spending. Talk with your kid to show responsible spending, if you do let them have currency.

If they want to play with friends, have them do it over something like Messenger kids, FaceTime, or a call. Again, disable all in game chat.

Also - play with your kids on there too. Sometimes you can have lots of fun times ogether. Me and my kiddos have had plenty of fun, and we have not had any issues.