You may already be aware of the online game, ROBLOX, and the possible safety concerns surrounding it. However, if you are not, then a quick look at www.net-aware.org.uk/networks/roblox/ will be of use.
The concerns are that there is open chat and opportunities for adults to be in direct contact with children and that inappropriate images can be generated by game players. There appears to be ways to put 'safety locks', access restrictions and limit expense, but it is unlikely that children will choose to do this on their own. Please be vigilant and check what your children may have access to.
Other Useful Links:-
NSPCC Net Aware - Do you know what your childre are accessing online?
DIGIDUCK - An online E-Safety story to share with children and young people.
Cyberbullying - Information for parents.