Gustav Mahler & Children

Gustav Mahler was one of the few composers who used Children's Chorus in his symphonies. 

In his third symphony, in the fifth movement ("What the Angels tell me") the children's Chorus enter singing "Bimm, Bamm" accompanied by bells. In the fourth symphony, though children's voices are not used, a soprano sings about a child's view of heaven. 

In the eighth, "The Symphony of a Thousand", in the first part "Veni Creator Spiritus", a Gregorian Chant in Latin, children sing "Gloria sit Patri Domines" (Glory be to the Lords) & in the second, set to the final part of Goethe's "Faust", they sing "Ich spur soeben." (I am just tracking).

In 1904, he composed "Kindertotenleider" "Songs on the death of children", despite his wife Anna's pleading not to tempt providence. In 1907, his daughter Anna, aged four years, passed away due to scarlet fever, justifying her mother's premonitions.