How much time should be spent on vocal warm ups for children's choir, aged 9 - 13, in a 30 minutes lesson?
This is a question where even professional choir directors and singers can't agree on. Some do quite long vocal excersises (sometimes with all kinds of gymnastics) and some just start to sing.
The excersises are not so much about not wrecking voices but more about getting in the right set of mind for reheasing and 'tuning' your vocal 'instrument'. It allows to leave the busy world on the other side of the door.
If a singer has a bad technique then the warming up excersises won't help him/her not to wreck his/her voice.
I prefer a short warming up period (no more than 5mins) to get into the spirit of the rehearsal. But as you have only 30 minutes with the kids, I would suggest to start with a known song or to teach them a special song for warming up.
When you use a known song, they can look at you all the time instead of their scores and you can then sign them to sing louder, softer, faster, slower,... That way you'll teach them to follow the conductor, get them in the right set of mind for the rehearsel, keep it fun for them and combine warming up with another excersise.
Most of the singers will be warming up during the first 5 minutes, so rather than waste (if that's the right term) those 5 mins, just get into the singing. Only if there's a great range of vocals or volume changes, which may be unusual, just 'get on with it!' Right or wrong, I did it for 30+ yrs that way.No warming up, straight into singing. Don't believe I wrecked anyone's vocal chords, certainly hope not!
I am both a classical tenor and previously trained many children's choirs.
In my experience, warm up is often a waste of time. Even for me.....
(However, this depends on using good singing technique...opera singers who use a lot of vocal fold thickness use the muscles in the throat more aggressively, and do need to limber these muscles to avoid straining. Opera singers who use a lyric technique (less thickness) will sing only a little warmup, and mainly to re-establish their technique for the upcoming session of performance or practice. I no longer warmup.....I just sing).
Children's voices are very light, and if they are taught to sing properly, warmup is not necessary.
The indicators for good singing at all ages are even vibrato (never sing with a straight tone), absence of breathiness (almost always a problem with choral training), sufficient volume (children are often taught to sing too quietly), and very accurate and overdone diction.
A well-trained children's choir will match vibrato (rather than blending using breathy production, as is usually the case), and will be heard over an orchestra playing softly (most children's choirs would be inaudible without microphones).
Once children know how to sing properly, the best way to begin a rehearsal or prepare for a performance is to sing through something very easy that the children already know, and then compliment them on their execution. This will bolster their confidence and enthusiasm, and also remind them of their singing technique.