I recommend starting with piano lessons, and let them play on the organ at home to get them interested.

Many universities have active jazz programs, and they may be able to hook you up with a MM student or professor who can teach younger kids — if there’s one near you. Also check your local sheet music shop for teachers. Sometimes they even teach at the store.

IMO, a good age to start lessons is around 5-6. At 3, the motor skills might still be rough... but again, speak with an experienced teacher.

I’m classically trained on organ, but I do play some jazz piano here and there. I can’t play jazz from leadsheets, though, because I wasn’t really interested in jazz until my 20’s. I need the chords written out. It’s important to develop the ear for that style from a younger age, so shop around for the right teacher!

But either way, you can’t go wrong by starting out with basic piano lessons. The piano is more intuitive and approachable for beginners, and the muscle memory will transfer from “intermediate piano” to “beginner organ” nicely when it’s time. That’s the traditional approach, AFAIK.