You can't take away smartphones completely. And once there is the smartphone a communication app is the most useful thing they can get. Better than any game or whatever. But an open messaging app can quickly lead to issues.
Would be good if there were non-profit vendors of similar things, bit they don't exist, yet.
Here's an alternative: restrict usage to a simple time-boxed window each day, contingent on the completion of what the child needs to have done (chores, homework, etc.). Augment that by limiting what apps the child will have access to, based on what is reasonable at the time. It requires effort, but seems totally reasonable. There doesn't seem to be a strong argument for children having continuous access to devices that have empirically been shown to cause serious mental health issues, not even including the crazy dangers of the internet (stalking, child sexualization, etc).
Alternatively, don't control anything the child does and hope it works out.
Back in my days it was typical for us to run to the phone booth "mom, I'm still out playing, will be late for dinner" Now there aren't phone booths anymore.
The concept of the kids messenger by Facebook sounds right. Quite risk free communication, which allows giving the kid freedom without having to always look over their shoulder what they are doing.
As said: that being not from Facebook would be great!
Someone mentioned the other day on HN that Social Media is the modern-day Smoking, and I agree there are a lot of parallels. Surely we can keep our kids out of S.M. without restricting them from using other, less harmful parts of technology.