There are all sorts of devices so you need to check what your device can do.
Young children tend to move unfocused over the screen, so you may need to change the device's swipe functionality.
For children who like to press the big home button of the device, it's a good idea to change its settings. It prevents the child from accidentally leaving the app.
There is a handy functionality called 'Guided access' in Apple iOS and 'Fix Screen' in Android that give more control possibilities inside apps.