an idea suggested by @psanketh
I frequently see vans at school picking up elementary school children and taking them to different activities. Personally, I am never comfortable with anyone other than my husband or I picking my kids, but I am sure the anxiety factor is fairly constant for all parents.
So imagine if the kids could just press this device in their backpack or so to let their parents know that they’ve reached class. Even better, keeping my absentminded son in mind, perhaps there is something inserted in his shoe that notifies me when they reach a particular address. Or maybe he jumps twice and I get notified or I can always see where he is when I open my app etc. but I think you get the idea 
This is similar to something Faramarz said in Check In #2 - about knowing when his daughter got to the library. Press the button to signal, “I arrived”. It doesn’t require extra communication that phone calls and text messages often imply.