I would suggest starting with carefully going through the Quick Start Guide and not using the app to begin with. Get those 3 controllers and 3 cars connected to the powerbase and running as they should - check the lane changers are working, go through the pit lane, use the brake button and then have some fun. If anything isn’t working at this stage, let us know and we can help problem-solve.
Once you are 100% sure everything is working with just the powerbase, launch the app. Run a quick race with no features. The Calibration is to set the lowest speed for each car to use in the various in-race features. Follow the instructions on screen, the process has changed slightly from the Quick Start Guide.
The main thing you need to know about using the app is that each car must be set up in the app to match the colour that car is connected to on the powerbase/controller. I call it the rule of 3 colours. I recommend the 3 cars are connected to the powerbase as red, green and blue. When you open the app, choose a 3-car set, such as the Sunset Speedway (it is in the latest iOS ARC app update) or Triple Cup. Those will default to 3 cars in race set-up, with red, green and blue edges to the tiles. Match those tile edges to the colour each car is connected to on the powerbase and you are ready to race. If the colours do not match, the app will appear not to be counting laps, for example.
Once you have this basic race set-up sorted, then you can gradually explore and learn how the more complex features of the app works. My personal opinion is that it is pretty intuitive once you have the basics clear, especially for young people. If after a session of exploring and racing you have questions or problems, ask here - there are enough of us around who have all got to grips with the ARC app and have our own ways of doing things.
I hope that helps.