Is there value in teaching hymns to children? Are they effective teaching tools? How do you incorporate them into rehearsals? Here are a few thoughts:

Why teach hymns to children?

Church heritage

Connect the children to the church's history and invite them to be a part of the legacy
Help develop their identity as a choir. How is a church children's choir different than an elementary school or community choir?

Spiritual formation

Connect the hymn texts to Scripture verses
Discuss the meaning of the text and communicate its relevance for today

Music education

Use hymns to teach children about singing: unison, 2-part, canon, vowels, consonants, shaping
Identify elements of tonality and rhythm once the hymn is learned: recognition of patterns, phrase structure, meter, melodic lines
Introduce the children to the various genres of music represented in the hymnal: spirituals, folk melodies, etc.

Considerations

Hymns in rehearsals

Choose a hymn to use as a gathering or welcome song
Select a hymn of the month (I tend to use only the first verse)
Connect hymns to anthems and other special music with a common theme or musical element

Hymns in worship

Use hymn arrangements as anthems
Have the children lead the opening hymn for the congregation (from the front or while processing in)
Give children an opportunity to be active participants in the service