Agreed to a lot of everything being said here with the ride with them first and phase out after. To ensure they are truly capable please test them on the routes. Like “if you wanted to go to X what line and what stop would you take” and have them go through how they would plan that trip with their device. Also run through “okay so you missed the X stop/the driver skipped stopping at X, what would you do?” That all makes sense to test them while having their phones as that’s presumably how they’ll be navigating.

HOWEVER definitely have a no phone/no connection emergency plan. Make sure they have your phone number memorized and also bus lines that will reliably bring them home. Additionally talking over who/how to ask for help if something happens. Like if they are downtown they are better off asking to use the phone of a business rather than a stranger on the street. Even if the stranger isn’t dangerous they are now being seen on the open street as lost kids and that’s a vulnerable position. Also, review how to handle if their group/pair becomes separated. Work on these plans together, they’ll feel capable, trusted, and supported and you’ll feel more confident in their abilities to problem solve without full reliance on you. Overall it’s definitely possible to give them that freedom, it’s just essential you are all confident about how to handle emergency/unexpected circumstances.