My kids have both played on Roblox extensively, and I'd say their experiences were almost entirely positive. Both started out by playing with their friends, namely they and their friends had accounts and their avatars meet up on a particular server. They would chat with each other, go visit each others' 'houses', play games, and generally explore. That was the only way my daughter used it, and she has moved on to watching Youtubers, which actually is less interactive, in my view. The neat thing with Roblox is that it does have the creative aspect, so one of my son's friends set up his own server and three or four of them would meet up, build an airport or other structures, and generally have fun. Also, kids who are interested can design their own games. Our son is really interested in planes and flight, so in middle school, he got involved with some Roblox groups which set up model air forces or transportation group, flew 'missions', did virtual airshows, things like that. It was actually kind of cool because he was interacting with people from various parts of the world and it taught him alot about the politics of how social groups work. Now that he is in high school and busier, he's pretty much stopped playing Roblox, as far as I can tell, aside from an occasional visit to touch base with two friends whom he sees rarely now.
Sometimes obnoxious kids do show up on Roblox who try to throw around bad language, but there is a censor function which pretty much blocks it. I hadn't heard about the chat feature issues, however I could imagine that happening. Given that your kid is pretty young, I'd suggest that he can use it as an additional way to play with friends outside of school, so pretty harmless.
