From what you've told me, if it were me, I'd teach the whole group both parts. Spend an equal amount of time on both parts. I'd divide the group into two equal sections only after everyone felt very, very comfortable with each part when sung on its own. Two part singing requires much repetition on the single lines so that when two parts are put together, each group can sing with equal confidence and not be thrown by the other section. Dwell on the helping the children to really get to know the music, rather than on you spending time listening to individual voices. Teach the words and the music by rote, and divide your chorus into sections only when the group is quite ready.

Any other ideas/opinions from other 'catters....?

~black walnut