Yes there will be some problems created from them having devices, but parenting isn’t supposed to be easy, it’s supposed to be educational and supportive for the children. Which forced abstinence is not.
Do they know you do this? Otherwise this seems like a very effective way to create trust issues in your kids.
For us, it’s a system that’s worked well. So well, in fact, that our kids have felt comfortable coming to us when they see something concerning in a group chat rather than waiting for us to find it. And in return, we’ve learned to trust their judgement a lot more because they’ve demonstrated mature behaviour online.
I trust my own children but you’re right that I cannot guarantee that they’re not bullying others and deleting those messages. However I’d hope other parents are monitoring their children’s phone usage and would tell either me or the school if my child was causing issues. That’s how a healthy community of parents are supposed to work.
Also your comment has a tone of “kids can find a way to bypass parental oversight so why bother parenting in the first place?” I don’t if that is intentional or not. But it’s an attitude I have seen other parents adopt and, unsurprisingly, their kids are usually the little shits that cause trouble because they know there are zero repercussions.
At 11 I wouldnt expect them to have unsupervised internet access. At 16 I might, but by the time they’re 16 I wouldn’t need to monitor their online activity so closely because they’ll have several years of trust and experience built up.
Obviously, this'll have to change at around 16, but those conversations need to happen anyway.