They wouldn’t need the app on their phones unless you want them to be able to turn the lights on and off with their phones. If you have a voice assistant like echo or Google home, they can use that. Or they can use the wall switch.
There are also a number of handheld buttons and remotes that are popular, particularly for a nightstand or coffee table. You still need the relay to actually control the in wall circuit, the button is just a remote for that device.
If you do want them to be able to turn the light on and off with their phones, you either give them the SmartThings app or you can give them a dashboard app. Action tiles requires a paid subscription, but is very popular. It lets you customize exactly which devices appear for each person, so it’s an easy way to make sure that one kid can’t turn off the lights in another kid’s room, for example. 
There are also some free dashboard versions created by community members. You can find them on the quick browse lists in the community – created wiki in the project report section on the dashboard list:
So there are a number of options depending on exactly what you want.