> I find it kind of hard to believe that they just happen to have location sharing on without any sort of incentive

The incentive might be as simple as not being bothered by a phonecall from a parent.

I'm sure most kids would be delighted to wear a brick if it reduced the stupid "where are you?" phonecalls by 90%.

Or even a small bacpack if it had a satiety sensor and cut out "have you eaten?" phonecalls as well.