χαιρε, ω Ρηφε ο φιλος μου!
If you are still looking for materials, the Greek resources at Classical Academic Press (http://www.ClassicalAcademicPress.com
) may be helpful. I haven't used them but I saw them in a catalog and they seem to have a good a approach. Their materials are aimed at homeschoolers, so probably they would work well for a church setting. If you write them they might send you some samples. There are also a few samples on their website. They have been building a good selection of Latin resources over time, and hopefully will do so with the Greek as well.
I haven't gotten to teach Greek yet, but having taught some Latin to kids (K, 1st and 3rd grades, 6th grade, 7th grade), I can tell you that using TPRS and songs, games, and lots of visual aids, props, maybe puppets, multimedia, skits/drama could help, also speaking the language as much as possible. I expect these things would work with teaching Greek to kids also.
I hope you'll share with us how things go. It's exciting that you have this opportunity.