Is it true that there is an alarm that only kids can hear?

Yes, As people grow older their ability to hear very high pitched sounds (such as bats and silent dog whistles) drops off. This is partly due to aging deterioration and partly due to the fact that the structures in the ear that detect the sound get bigger and thicker.

Thus it is possible to build a sonic alarm which emits a loud sound at a pitch that only children (and some lucky adults with good hearing), dogs and cats can hear.