If the device is running iOS6+, Apple introduced a feature called 'Guided Access', which is available in 'Settings -> General -> Accessibility'. The feature introduces a method of locking the device to a specific application, as well as limiting touch interaction based on user-definable touch locations. The feature can be PIN protected, and when enabled is activated/deactivated by triple-tapping the Home button.

NOTE: if the device battery is fully drained, when power is reapplied, Guided Access mode does not automatically reinitiate.