Although I agree with you that the changes suck I believe they are also going to be coming to the Galaxy S8 (Not sure if you are on that one now). Once Oreo is available for all of the S8 line it will also come with Experience 9.0 which most likely is what has changed everything with the UI.
Install Google Allo and Duo on the kids devices and then on your phone. You will be able to message them with Allo and also video chat with them with Duo.
The white headset does not come with an adapter. I am looking at one right now. The one that came with the S8 does have an adapter for the VR headset and that one should work without issue.