I require a couple parents to stay in the room to help with crowd control.
If you have a school, you will usually find that some of the parents will have their clearances because they are teachers or aids, or coach CYO basketball or scouts or were chaperones on a field trip.

Thanks for the quick replies. We rehearse in the church sanctuary in the choir area near the organ. In the past I have had a few parents take turns staying at rehearsal, but haven't used any parents once I had my assistant. I seem to recall that all of the parents who used to sit in were also PREP teachers and so had clearances.

I'm confused in that my priest changed his mind from what he told me a few weeks ago. My longterm assistant, after she got sick, was replaced by one of our good soprano adult choir members who did the whole Advent season with me. However, she didn't have clearances. But, the priest said it was ok because I was there (and do have them). Now, today though, he tells me that anybody who helps will need to have them, so that confuses me as to why the change? He does change his mind rather frequently sometimes though on certain things. Surely makes it harder to find someone. A quick search on this DOES show that any adults working with kids must all have clearances, so what we did with the substitute assistant was fudging it. I believe he thought it was ok because that lady also worked in a local community theatre with children, although I don't believe she had clearances. Moot point since we got through Christmas Eve and she said she can't help going forward. As it turned out, she wound up in the ER on Christmas Eve morning and so I had to scramble to find a third person to fill in! Luckily, another of our adult choir members is a public school music teacher so no problem.

As for recording vocal lines, we rehearse with my laptop using internet links to our arrangements, so the kids hear all the parts that way. Then, later in the rehearsal schedule, I substitute the laptop for recordings of the accompaniments that I did previously on our organ. So the other part of the assistant's job is to direct the kids during anything that I'm accompanying on the organ.

Regarding singing their parts...if you have older girls (or even a strong boy soprano) in your choir, you can have them sing the parts in the treble range for the younger kids. As a soprano, I have this difficulty directing middle school boys, but in reverse. If the boys aren't intuitively singing down an octave from me, and I can't sing in their range, I have one of the stronger boys sing it to demonstrate.

I now teach in a school, but in my years of directing parish youth choirs I had a volunteer assistant. She did not have musical skills, but was great with kids (retired teacher). She took care of behavior issues if they arose, helped pass out music, helped with snacks, and made sure all the kids were picked up afterwards. Volunteers like this are worth their weight in gold if you can find one!

Church or school, I don't think it makes a huge difference as to how you motivate the kids. They have to want to sing what you are asking them to sing and they have to want to listen to your instruction. Finally, you--not they--are the adult in charge. You have an excellent background in elementary band. Don't feel shy to pull tricks out of your hat and see what works.

That said, here's my choral specialist brain on the matter of assigning parts in children's choir. With kids up until about the age of 11, there's pretty predictable developmentally appropriate singing ranges. After that, vocal categorization begins to make sense. However, I'd still shy away from siphoning kids into fachs, as their adult voices won't settle until their mid-20s. Nothing makes me roll my eyes like the audacity of a 16-year-old boldly telling me, "I'm an alto." Between the ages of 13 and 23, I sang (in various professional and semi-professional capacities): soprano 2, alto 1, alto 2, soprano 1, alto 1, soprano 1 again, and alto 1 again. It is healthy for kids' musicianship to sing a variety of outer and inner parts, and using the verbiage of "you are singing the alto part on this song" rather than "you are an alto" may help them get over their resistance to singing a different part.

With the age group I spend the most time with (8-13 year olds), I divide them by birth month and adjust as necessary. There is an immediate sense of buy-in when they are placed on a "team" with other Jan-June/July-Dec babies. ("Raise your hand if you were born in _______ months. Okay, you're singing the soprano line. Go!") I've never had a complaint. If one part is decidedly weaker than the other, I'll move a couple of strong singers around.

I believe a combination of affirmation and gentle challenge can be effective. If you feel a child would be well suited to the alto line, you might encourage them by saying something like:

“Many singers can carry the melody, but singing a harmony line requires strong musicianship and careful listening. I think you have the musical ability to do that well, and I would really value your help on the alto part.”

You might also frame it as an opportunity for growth:

“The melody often feels comfortable, but harmony can be more demanding. I think you’re capable of handling that responsibility, and I’d like to see you try.”