Is there no adult you could get to come in and assist you? Maybe a director or musician from your church or a local church in your network of acquaintances?
A large part of the problem is that kids, in most cases, have a hard time submitting to the authority and leadership of other kids.
And I totally cosign the stuff Jonathan said as well. Get rid of the dancers and definitely have a discussion with them. In fact, instead of rehearsing next time (unless you have an engagement coming up and can't afford to skip rehearal), you should have an open discussion with them. Start by asking them what they want from this experience, why they joined, what they like about it, etc. Then, share your concerns and allow for feedback (but be sure to keep control over it). In the end, give them your expectations and ask openly and honestly if everyone there is willing and able to live up to those expectations. End by encouraging them somehow. Maybe let them know how well they're doing vocally or that you appreciate everyone's timeliness, or how much they've improved in a certain area. Mention an upcoming engagement if you have one, and if time permits, close with a run-through of an upbeat song that they usually enjoy singing.
You don't want them to leave discouraged.
(And btw, until recently, I was directing a choir full of college students as one of the outreach projects at my church. One method I used that helped with excessive talking is to just be quiet while they talked. I'd get a very serious expression on my face, and sit there with the "I'm waiting" look. It generally only took about 30 seconds for them to stop, sometimes less. It's a lot less chaotic than yelling and a lot less demoralizing than lecturing or fussing. It's pretty effective with that age group.
