I would have loved (and hated) this as a kid
If their blood sugar was 60 mg/dl (3.3 mmol/L) then there's a chance that they could have been a bit disorientated and that some light assistance (a phone call from dad) is what's needed.
The device should have given an alert, but they could have missed it - a replaceable sensor unit is adhered to the skin and is paired with a larger more power-hungry "base unit" which is carried elsewhere (e.g. in a pocket, but maybe they had it in a bag, and so missed the alert). And they might not have missed the alert, just not acted on it quickly enough, or they might have been a bit disorientated. There could even just be embarrassment at having dad calling to make sure they're okay.
The point is that the device gave them a backup - someone to help when they were in trouble - and that's an incredible thing to have.
I've been diabetic for >30 years now (since the age of 11), and the simple fact is that diabetes is an absolute b*gger - it's complex, difficult, and highly personal. Dealing with a condition that is with you 24/7, and which requires very careful management and modification of treatment regime and medication every day is a huge challenge. Add to that the dietary constraints which it introduces and it's a big thing to deal with.
These kinds of devices which are able to export data over cellular networks and to trigger alerts to third parties are absolutely fantastic. I'm sure that my parents would have loved this when I was a kid. I'm sure that I could have hated it as well with it telling them when I was getting things wrong, but the benefits (not just the alerts - glucose readings every few minutes 24/7, giving incredibly rich datasets to assist in management of the condition) would have been huge. Effectively having somebody with me 24/7 to help when I was in trouble would have been the (sugar-free) icing on the cake.
As much as we may get stressed about the possibility of data getting to unwanted parties, it is impossible to ignore the health benefits and I suspect that for the vast majority of parents, users and medics those benefits will far outweigh security concerns. That's not to say that security shouldn't be a concern - it absolutely should - but other clear benefits will easily push it down the list of priorities.