If this is going to be an Android mobile game you would need to add the Touch plugin:
1. Click anywhere on any layout to add an object, scroll down to the Input section and select "Touch".
2. In the event sheet associated with the layout containing the hidden objects, click "Add Event" and add a Touch condition.
3. Choose "On Tap Object" and select the hidden object to set the touch condition.
When testing this on a computer make sure you have "Use mouse input" ticked in the Touch object's properties - it will then treat the mouse input as a touch input.