"O blessed are the poor in spirit" — an Orthodox Kontakion set to the Beatitudes text by Richard Proulx, found in the Worship Hymnals: — Worship III, #620 — Worship IV, #738
I've done it with children. It's very simple and easy to pick up.
There are actually several settings of the Beatitudes on the Hymnary website, most of which are in Anglican chant (instead of strictly a hymn).
There's one with a refrain from a "Junior Hymnal" (composer uncredited) that looks like it might be fun. I also like the Turle setting, a double-chant.
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