The LEGO EV3 Mindstorms robot is a LINUX box. A LINUX box on which you can run an alternative OS available at https://ev3dev.org. An alternative that is now sanctioned by LEGO itself with the introduction of microPython. One of a dozen or more text based programming languages you can use to teach coding on the EV3 robot.
You can use most of these languages to build a controller. A controller that you can use together with this app to remotely control your LEGO robot. Note it will need a WIFI chip.
You can find examples of the driver code youll need to install on your robot on the app website. A remote that can be customised to fit your needs as illustrated in the screenshots. The port labels and buttons are customisable, you can change the text, the color and using accessibilities features in iOS the font size. You can also hide and show buttons and ports as needed. You are in control of what happens when different buttons are pressed and you are in control of how frequently updates from the sensors are sent back to the controller.
Using peer mode in version 2.8 you can also use multiple iPads/iPhones to control your robot. Note the arrow used in the peer screen was downloaded from thenounproject, the graphic designed by sugeng hariyadi.