It would be great if you could block single player apps! Unfortunately, the kids will sooner or later find out that they can download the packages from the browser and propagate it directly among group of friends even if most of them had it blocked.
On desktop, a workaround is as simple as installing or configuring a client in a way that it does not have access to the GPU or is buggy, because the sandbox window will then fail to open.
You can disable the WebView service (and a few more) on Android to get a similar effect. I think installing a screen time logging app and agreeing upon ground rules would be more productive in general (i.e., x minutes of gaming after they finished homework or chores, etc).
You could set up your own chatmail server that blocked all large attachments, such as those above 200kB (or even above 4kB if you wouldn’t like to allow using images either) or perhaps filter the network connection in a similar way.
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