Ask HN: Hackable Phone Watch for Kid without app

I'd like to equip my kid with a watch that can be used as a phone.

I found many different model, but they all require an app to operate.

So I am wondering if the HN community could help me find if there a model that is more "hackable"?

For example, for GPS tracking, I'd like to be able to specify an URL and have the phone do a POST request on it every few minutes or so.

I only need phone (audio only) and GPS tracking. And I want to use my linux workstation to configure it (like updating configuration file or the like).