Deactivate the Bomb Game
The concept: Children are given an arduino with a screen of some sort or a piezzo buzzer that is counting down. Higher frequency of buzz means another minute went by.
To deactivate the countdown: they need to build the right circuit. This can be something like doing a light display.
Children will need to follow clues to find the parts and assemble. Team work will probably be involved.
BTW, nothing happens if the time runs out.