The challenge is to help the children focus their tone quality to sing beautifully and on pitch. When they all sing poorly and out of tune with each other, they cancel out their sound and it just becomes indistinguishable noise. The director can help by choosing material that is geared to the particular children's strengths. It is thought that most children sing best in roughly a one octave range, Middle C up to Treble C. There are many children that have wider ranges than that, but as a whole, this is a good, accepted range for most children.
Children learn easily and best when the song is modeled for them with proper singing. They seem to effortlessly mimic how the vocal sound is produced and the inflections used. Many kids have trouble matching pitch when the song is modeled by a deep, low voice in a deep, low register. Better to have the model sing in the Treble clef with a light, child-like quality. It can be amazing how fast children can learn the melodies and the words when given a good model to follow.
The actual Warm-ups to use can be varied, but should consist of scales, and skips, and motives up and down in their range. The best ones have little nonsense rhymes to go with the notes such as "Mommy Made Me Mash My M & M's" coming down the 5-note scale. The repetitive M sounds help focus the placement of the vowel and encourage singing in tune as well as using good breath support. Besides, a fun, non-sense rhyme is much more interesting to sing than just vowels! I have collected other fun Tongue Twister Vocalises you could try. A good few minutes of vocal warm-ups can improve the energy, focus and polish of any show!