We got 3 cellular watches for our kids and set them up on one parent phone using the family connection thing. Each watch used only the phone number assigned to it, for both phone calls and messaging. Maybe they do things differently if you set it up in that way.

As an only device for kids, it was about 80% there and over the course of a few years Apple made 0 progress, so we gave up on it as a concept. Only 1 kid still wears the watch; the other two are e-waste.