Sure, but knowing kids they will find their way around it. You're starting a cat 'n mouse game.
But this should do the trick. Create a group policy (Network Wide/Group Policies) with some L7 firewall rules and apply it to the chromebox (through the clients page). After creating and applying it, disconnect and reconnect the chromebox (or your iMac when testing) to the network.
Because the policy is only applied to the chromebox, their own iPhones and XBOX's should still have access whenever your existing control system allows them to.
I added ttvnw.net just in case. That seems to be the domain name from which the streams themselves originate.
More background info here:
https://documentation.meraki.com/MR/Group_Policies_and_Blacklisting/Creating_and_Applying_Group_Poli...