Play the lowest note in the low part. Ask the children to sing it. Those who can, raise their left hand. Now play the highest note in the high part. Ask the children to sing it. Those who can, raise their right hand. Have any children who are only raising one hand to stand to the left or right. Divide the children raising both hands in such a way as to make the groups even. Children raising neither hand, treat as BW suggests above.

It's a thought.