Go to a school in your area, meet some teachers, maybe a music teacher and get acquainted with them. Then tell them you can take kids to the pipe organ in the church nearby.
If you are not an organist of that church, go ahead and meet the real organist and tell your plan to him/her (that you want to demonstrate this instrument to school children). Most organists who are conscious of the state of their profession worldwide will agree to let you play for 30 to 60 minutes (those who are not conscious, well, you don't need them anyway).
And you don't have to be a virtuoso to demonstrate the organ. You can improvise, play some familiar tunes, hymns, songs that the kids would know and enjoy.
The single most important element of your demonstration should be storytelling. You should tell stories about the organ, its construction to the kids that would resonate with them. Open up organ panels, show the pipes and bellows and organ mechanisms. People of all ages love that tech stuff.
Make the demonstration less about you but more about them. Let them touch the keys and pedals, even play, if they can. Let them play especially, if they can't!
And don't forget to take pictures. Lots of them. In fact make a video or two of your event. Put them online and watch them spread.
Not every demonstration will be equally successful but if you learn from your mistakes, adapt your stories to certain age groups and improvise, little by little you might have the impact you seek on a new generation of organists.
[Thanks to Djuvaldric]
Practice the full perfect cadence in E major. The chords: I-V6-I-N6-I64-V-V7-I (see video example below).