Well, I learned many hymns and ‘religious’ songs in public school choir (back in the day!) and that is how I learned to sing ‘parts’. Something strange to me when I started singing on a ‘praise team’ in church is that the singers did not use sheet music or read music at all! They use chord sheets, and they know the notes of the chord. That is still a foreign concept to me…though I have learned enough to wing a harmony. But I am not as adept at this as my fellow believers, who learned to sing this way.
But, as far as teaching parts to children:
What we do is learn the melody first. Then, the older children learn the alto part by my playing it several times and singing through it with them (alone). Once we have done that 3-4 times, they usually have it down well enough to sing the alto while the younger kids and I sing soprano (melody).
I love the idea Carolyn had about having someone recording the hymn acapella! We do use instruments at our church, but I feel that acapella singing is excellent training for teaching confident singing.
I read the biography “Bonhoeffer” a couple of years ago, and was struck by how his family worshipped by singing this way. His love of music and hymns helped strengthen his faith while he went to martyrdom for Christ. What a precious gift we give our kids by teaching them words of truth musically!