My children downloaded this a few months ago, it's extremely popular with kids their age and very similar to a Lego / Minecraft type platformer.
I was amazed at the instant messages asking for friend requests from various people and could type what they wanted without any restrictions, i refused to let them have it and deleted it, now this has come to light http://www.dailymail.co.uk/news/article-4159602/Girl-six-sent-creepy-message-app-Roblox-stranger.html
and it doesn't like they're the only one.
Girl, six, was sent a message on mobile gaming app Roblox asking her to call a stranger 'daddy'
You can read the full story on the above link but just a heads up to check the app and whats being said or do what i did and just bin it.
Some filters are in place but obviously still lots of issues with child safety
Hope it helps some of you anyway!
Also Turning safe chat mode on
Thanks to antony4462
Update 14th Feb, messages still being sent :/http://www.mirror.co.uk/news/uk-news/dad-roblox-warning-after-young-9821213