I want to set up a 'home phone' network for the kids in a couple of different households. The kids are too young to have their own phones, but are currently dependent on parents to contact each other in the way we did when we were kids

My idea at the moment is to set up an old Android phone on the wifi in each house with messenger or similar, then uninstall all other internet apps and lock down the play store

Does this sound like the best option? If there a better tool that is still cheap/free and will be relatively safe for the kids to use unattended?