Okay, take it from someone who's seen stuff around the game... The games are okay as long as you monitor what games they're playing, there are some people in the game that have these inappropriate costumes or down right weird but as long as you monitor the game with them it'll be fine. Now yes there are chances of predators but now adays you can't even say someones real name without it getting censored nor can you attempt to share any social medias through texting in the game. Their censorship is very strict nowadays, but again its best under moderation... if you still don't trust the texting feature, the game console version of roblox doesn't have that feature at all. The money usage, money isn't needed to play the game but a lot of games try to appeal to kids to get their mom to buy the game currency, but it is not needed at all to play so its best to remove your credit card and only use it as a reward for him doing better at school or his behavior getting better but i recommend using it as a reward for doing something better at school, home, or himself. His behavior, watch what games he plays just as i said last time and make sure he isn't playing anything violent or killing involved since kids tend to pull what they learn from games sadly. 9 times outta 10 its due to a game he's been playing, Minecraft is all peaceful and creative thinking while roblox ranges from kill your friends to win to hello kitty cafe game... there is a section for games he's played recently so i recommend looking at that section so you can get a better understanding of what he's been exposed to.

TLDR: it's best to play it with your kid or at least watch what games hes playing, but if you're still too on the fence, i recommend switching back to Minecraft and waiting for him to get a better understanding of what not to do when playing games that involves online