I wouldn't let any child near Roblox full-stop.
Only reason mine knew about it or went near it was the bloody primary school promoting it - then 12 months later screaming about safe guarding concerns and about lack of parental supervison (mine were supervised in a downstairs room and asked what they were doing periodically and we still has issue stopping DS using it) while still allowing school access before finally blocking it.
That was a few years ago.
They have cheap/second hand smart phones from 11 - as not having such phone at secondary would mark them out - as everyone does. Since this last year the also have own laptops as they've been needed for school work and since eveyone's been in the house they have got used to having them in their rooms to work.
Only eldest has a whatsapp group with friends - other two make do with texts and none have FB/twitter or other platforms - we reserve right to check what they are up to on the phones which they know and phone are downstairs ar night. We and the shcool and us talk about people not being who they say on internet regularly - even before the are on social media about employers and other looking at past posts - having things locked down.
We're unusally tech savy and seem to have good relationship with our kids but we've still had issues - DS managed to run up a phone bill depite a cap becuase he found a round about way to buy in game things and not realise he was being billed and we did have to pay that and then put in even more safe guards on the phone contract - that was a leaning moment for us all.
I think while parents have a part to play but everyone's very quick to blame them for any issues and I don't think normalising odd on-line behavior in national newspapers helps anyone.