This relaxes the singer, increases blood flow, and encourages healthy singing. Rather than bang out one part at a time, try playing two parts at a time. You'll be surprised how quickly they adapt to this, and how much more is accomplished in a rehearsal. I personally like to keep everyone singing all the time. For example, if I were teaching the soprano and alto parts together, I would have the tenors sing along with the sopranos an octave lower, and the basses sing along with the altos an octave lower.
Many choirs serve goodies at the end of choir practice.
This also ends rehearsal on a sweet note, and helps retain membership. Adding children to your choir poses several challenges. First of all, the child's voice until adolescence, properly used, has a range similar but not identical to the soprano. Second of all, children tend to have different pronunciation issues than adults. Finally, the vocal color is quite different.