for diabetes sensors you need to have a phone with android or ios to use the app which sends data to parents/doctors (stuff like dexcom)
If the other 99% of kids want their parents to gift them smartphones too, I suppose deliberately cultivating diabetes does show dedication to the cause.
Most of them have a receiver option too. Dexcom does. They're another cost though, and quite expensive for what they do (~$400 for the G7 receiver). They don't have Wi-Fi though, so they can't upload data (at least for Dexcom).
well in germany you get the dexcom "for free", but I'm not so sure about the reciever. my sister basically just bought an older used iphone for my niche and used that which was way below the price point and my sister also wanted to know the datapoints (and sometimes needed to sent them to a doctor) so the phone was basically a must.
Apple watches?
Lasts for less than a day, and are they independent these days or do they need a paired phone?
Nah...Apple Watch batteries last 2-3 days nowadays with the proper settings. There are cellular Apple Watches available that do not require the paired phone to be close to make calls, texts, etc.