If we're going to learn the chant ordinaries of the Mass, I am very much pushing the use of the Chant Tones in the missal first and foremost. Start with English (but always use the Greek Kyrie) and then also learn the latin chants, particularly the Sanctus and Agnus Dei. Agnus Dei Ad Libitum II is one that should get regular use. Then learn the Gloria from Missa De Angelis. Most parishes and places will not be able to go much beyond this, but it wouldn't be too difficult to ad the Kyrie from Mass XVIII for Lent and Advent and then maybe learn Mass XVII, Mass XI (Orbis Factor) and the whole Missa De Angelis.
Play or sing for the kids whatever you like. I think in this conversation we're confusing exposure with participation. If we're talking participation, I agree totally with the kindergarten teacher. She's been trained to know what her kids are capable of and so am I, so I'll elaborate. :)
If you want K and Pre-K to participate, try using music that is pentatonic, or if it is not pentatonic, make sure it only includes a range of a 5th. I have used with great success with K and pre-K old pentatonic Southern Hymns. For example, How Firm a Foundation, God of Day and God of Darkness...
You can also try teaching simple refrains of songs to the little ones while you tackle the wordy verses (or a child who is verbally skilled for his age, or an older child helper). For example: Hail Holy Queen Enthroned Above
Again, I also am against "watering down" the musical art we present to our young ones, but if we teach them from an early age not to sing or not sing well we harm them for life.
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