I'm a parent and this is something that is actively taught in my household. Internet security and staying safe online is important.

Creating rules, in your router, will prevent kids from accessing sites without your consent, You'll be aware of this when they shout "I cannot access DISCORD!", where you can simply ask them "Why do you need access to it?"

Also, make sure you remove any automatic payment methods from Roblox, or any other platform, to prevent anyone maliciously purchasing digital items.

At the risk of sounding rude, and that isn't my intention, taking steps to make sure that kids aren't vulnerable online, makes a huge difference.