If you can't run forums and chats because moderation is too expensive, then don't run 'em. Roblox is like the Pleasure Island in Pinocchio.
Though I think at least in Europe in the middle ages, 15-20 years old for marriage was more common.
That has (almost) always been part of raising children. The important question is, who has control and with what intentions? The intention of roblox they say out in the open, bind kids to their plattform, increase their engagement (addiction) and introduce gambling as soon as legally possible. And pretadors they don't see as a problem despite independent research very much say it is. Trusting them in any way sounds like madness or lack of care. So I am not in a position yet, that I am pressured by my toddler to allow roblox, but he is hooked on minecraft now. Just singleplayer, but also no idea how it works in that universe with public servers. I am not generally against the idea, that adults can talk with kids, but random adults from the internet? No way.
But this is not a Roblox problem it is an internet problem. And I do not know how to solve it except more censorship and control.