In the case of Roblox they have a horrible system where they estimate your age and only allow you to interact with people of a similar age, meaning if you verified your kid with your face then they'd only be able to interact with adults and not other kids. At least that's the theory. It doesn't take a lot of effort to figure out how a predator could misuse this system to their advantage (which is why I call it horrible)
Reference: https://en.help.roblox.com/hc/en-us/articles/39143693116052-...