Being a professional music teacher, I would start with Kodaly, but that's because it's what I prefer to use. Early emphasis would be on learning the major scale from DO-DO using Curwen signs as well. I would also be emphasizing proper breathing and phonation to ensure good tone quality and efficient use of breath.

The actual plan would depend greatly upon for what we are preparing. If we're preparing to sing for Mass, the repertoire would be something like this:

OF

Graduale Simplex for Proper of the Mass

Gregorian Ordinary, perhaps Missa XI

All of the sung dialogues with priest

Polyphony would consist of S,A parts of hymns either in English or Latin. I have a copy of the St. Gregory Hymnal, so I would use repertoire from that. Having a version of Salve Regina as you mentioned is a good idea.

EF

Graduale Romanum preferably for Proper of the Mass, however the Rossini Propers work as well if the GR chants are too difficult. I would, however, try to teach the children at least one of the chants from the GR, and perhaps use the Rossini for the rest if I had to do so.

Gregorian Ordinary, perhaps Missa XI

All of the sung dialogues with priest

Polyphony would consist of S,A parts of hymns in Latin. Again, I'd likely use the St. Gregory Hymnal for this.

Each of the 11 sessions would begin with solfeggio warm ups, followed possibly by range expanding exercises. You'd have to be careful with the younger students when doing range expansion, as they may be in various stages of vocal development and might not all have the same capacity for expanding at the time. Your 10-11 year old students might not have the capacity for a full child's range yet, but should be able to sing up to F5, and down to A3. At 12 years old, they should have the capacity for the full child's range of G3-G5, depending on the level of prior training and experience they've had. Your adolescent children will have voices in various stages of change, and that will affect their range capabilities significantly, especially the boys. Just work with what you have and expand gently.

Then, I would follow by working on chant and sung Mass responses, and then the polyphony and/or hymns. I would set aside some time each session to work on a little bit of everything. The last two sessions, however, I would plan to run everything in order, and I would sing the priest's parts in rehearsal when applicable.

When dividing up the parts, make sure that the children singing the alto part can match pitch and sing their part independently. How I have tested this before is I will work with the students on the melody for a while, and then one day before I've begun rehearsal, I will warm the students up, and then give them a pitch and tell them to sing the rest of the part. If they can do that successfully, then they're ready to take on an inner part. I've had situations where I haven't had anyone singing the alto part, and that's totally ok! Singing an inner part is an abstract concept and students don't really reach the psychological stage where they can fully grasp abstract reasoning until 5th or 6th grade. Your older students should be capable of singing inner parts, and some of the boys may be able to sing tenor or even bass parts.

Of course, this can go on and on. You can have the children rearrange themselves into the minor scale by putting the three top children at the bottom of the line. However, you will need to adjust the interval names appropriately.

Introduce the whole scale on day one.

I second Kathy's idea of introducing the entire scale on day 1. Also, please, for the love of St. Cecilia, don't teach them (even inadvertently) the mythological importance of the name of the syllables! Intervals matter more than names of syllables.