How to stop daughter's phone with own account running controls on my Mac

My daughter's iPhone (8 years old, separate account m.***@iCloud.com) is controlling the audio (play / stop) on my Mac (d.***@iCloud.com).

While watching YouTube in Safari it kept starting and stopping. A minute later my daughter came to me and asked me with "this" is on *her* iPhone, "this" being the play controls on *her* lock screen. When I pushed play, **MY** video on my Mac resumed. Pushing "Pause" on her iPhone stopped the play on my Mac. I restarted the phone and it repeated.

How can my daughter control my Mac? I am the parent on her account, but otherwise it is her account.

And more importantly, how do I disconnect her phone from my Mac?

And can *anyone*, i.e. any random user, run the video player on my Mac? Isn't that insecure?