We put an AirTag in our kid's backpack. That way we know when the bus left the school and when to go to the bus stop. Don't know what we'll do when our kid is more independently mobile...maybe an Apple Watch for real time communication over wifi.
You could give them a phone and ask them to call you when they get to the bus stop. Consequences of not calling you would be them getting board and having to wait for you to arrive.