The children should be able to understand from your demonstration that the organ operates not from hammers hitting strings the way a piano does, but from wind passing through pipes of different lengths. Therefore, the organ is really a wind instrument rather than a percussion instrument, so it belongs to a different family from the piano. That's right, keyboard. The keyboard on an organ has the same white keys with the same A, B, C, D, E, F, G names and the same pattern of 2's and 3's for the black notes. But there are also lots of differences.
Organs often have two or three, or even four keyboards, which organists call manuals.
Organists are very, very busy with both hands and both feet at the same time. Bach worked in churches for most of his lifetime, and he had to write brand new pieces for every Sunday in the year. A fugue is a piece that is built up of single melodies in different voices, as you'll hear. Listen to this piece as you close your eyes, imagine you are in an enormous building or church, and see how the sound makes you feel. Play the recording.