If their voices sound as good as I'm assuming, then harmonizing His eye is on the Sparrow, or Take courage my Soul (encourage my soul to some) would be nice with them. Take courage my soul sounds nice acapella, but for his eye is on the sparrow, you will have to get creative. I can imagine soprano, alto, and tenor parts, and the other two doing scat-libs (scats and ad-libs). Teens are creative (I would know, I am one) so ya'll just put ya'll heads together and tweak the songs.