I think you’re doing a great thing for this kid. The only thing I’m going to suggest is talking to her on her first day of work, when it’s just you and her. Explain that you consider her an adult, and you will cut her some slack at the start because she is in training, not because of her age. This means things like it is her responsibility to be at work for her shifts, to keep her phone in her pocket, to let you know well in advance when there’s something (school function, big exam, dance, or family function for example) that means she will not be able to work a given day, to call you herself if she’s sick and not let mom do it, etc. Kids may know the rules of employment in a superficial way, but they need it to be spelled out that they are absolutely going to be held to them.
Kids also benefit from having it spelled out that you’re going to throw a lot of information at them and you don’t expect them to remember everything, but you do expect them to remember there was something and to ask about it. Explain how you will handle mistakes they make so they’re not afraid to make mistakes, or to tell you about it. Stuff happens, and it’s how one deals with it that’s the important part.
This discussion will also help you mentally keep yourself in boss and mentor mindset, and away from Mom mindset. At this age a job marks a shift in how kids relate to the world and other people. They often appreciate being seen and treated as genuine adults when it’s made clear that’s the deal.