A help article notes that all Google Homedevices must be signed onto the same Wi-Fi network and Google account.
Android: Android 6.0 or higher; Google app 6.13 or higher; Google Play services; 1.5 GB of memory and 720p screen resolution; and the phone's language set to English.
Broadcasting starts rolling out today to the Assistant on phones and speakers set to English language in the U.S., Australia, Canada and the United Kingdom, with more languages coming soon. Just use Google Assistant on your smartphone to send messages to a Google Assistant-enabled device.
You might have had a home intercom system if you grew up between the 70s and 80s, and that's exactly the kind of functionality Broadcast offers.
Everyday messages can also be broadcast, such as time for breakfast/lunch/dinner, time to leave, and the like, which will be accompanied by a sound.
All you have to do is say, "Ok Google, broadcast it's time for school" and then all the speakers and phones will hear that message (well, provided they're already awake of course).