You can't run a public service without moderation, unless you want to end up with a toxic swamp. Definitely not a kids game. Typically it's worked around by not allowing player to player chat at all, or making it extremely limited, but even then you get workarounds (see dark souls with "try finger, but hole")

Moderation is trying to enforce some standards for the whole environment. Even if you got the tools to implement filtering for yourself, you're still sitting there with people who are happy with the abusive environment and will get through one way or another. And you get them everywhere around you. It's not something that each player can deal with separately unless they opt into private groups - but that's not the environment Roblox is encouraging.