I am a 30 yr old man with 2 children, and one of whom that is 8 years old. Not only for my son, but for his entire school, his cousins, and his family members his age, all of them adore Roblox. It seems absolutely harmless on the surface. I actually have spent a great many hours playing Roblox with my son and have first hand see why this game is anything BUT a kids game. Here are a few pointers I want to make for ANY parent thinking about, or probably blindly allowing your child to play this culturally iconic game.

Adults Play Roblox - Please understand I am not referring to adults such as myself who have kids and are spending time with their family. There are no joke 18+ year olds who have NO CHILDREN, and yet they literally play this video game as their preference. Never again will I judge the man who plays call of duty and follows the masses. The fact that there is a non youthful group, sitting inside, and playing Roblox is concerning. Many of them blast inappropriate music from various games. I have heard this myself in a variety of Roblox games in which one of the other players is pumping adult content music for all the little children to enjoy, profanity and all! There are reports of them playing Porn as well in order to troll.
A thousand ways to waste REAL currency - Roblox wants you to spend money on Robux. Not a big deal right? Whats the harm in microtransactions? The problem is that Robux couldnt be more worthless. There are literally traps inside each of the Roblox games which encourage kids to spend Robux, for literally NOTHING of actual value in the game. I recall one game where literally there was a giant, cool way for kids to spend Robux only for it to be a donation to the creator. There are a variety of items that wont even last 5 min sometimes costing $5-$10 or simply disappear after you log out. This is why your child is always asking for robux. You would get an equal value using singles as toilet paper as you would purchasing these items which are a snare for kids.
Adult themed video games - Roblox is a game which appears utterly harmless. And yet there are literally games which mirror Grand Theft Auto, and have your kids playing as criminals, stealing cars, killing police, and there is NOTHING you can do to block it. Some people may shout, THERE IS AN ADULT CONTENT FILTER! This filter is utteraly useless and when activated literally blocks 95% (estimated) of the games! Let that speak for the content in the BULK of Roblox games, but the Adult Filter is also so Draconian that your kids can not even play a spongebob game just because it has Lazers! Its utterly useless and nearly renders Roblox unplayable in almost every aspect.
I Could go on even longer, but I urge any parents reading this, take it from my who has been playing this game for months with my child only to discover a rather dark game which is a waste of money, makes Pokemon seem educational on the topic of Biology, and wants to expose your kids to a creepy group of adults, and a culture that wants them engaging in illicit activities and debauchery. You are literally better off handing your sub 10 year old a BB gun, or letting them play a war video game.